Hello everyone. 

So I'm crafting a campaign from scratch set during the spellplague. Although, near the "end" in the year 1393, and set in the stonelands/cormyr region. It is heavily homebrewed and lots of the content is non-canon (Arabel being vaporized, for instance). Most notably however- given this is set near the "end" of the spellplague era, magic has started to make a return, albeit in a new form. This is how I justify the reason my players have magic at all, and are able to wield it. But I still want there to be tangible effects from the plague lingering.

Anyway- My players enjoy a low-magic setting, but not a no-magic setting. However, I didn't think simply telling them  "spells fail sometimes" or "everyone rolls the wild magic table" (that might be expected in a spellplague setting) would be fun for the magic users in the group. Nor particularly interesting for that matter. Also- Spellplague setting stuff is kinda hard to come by, at least in my experience. And so, I crafted my own "condition" that magic users may contract as a result of using magic. 

I'll paste it below for you to read and even use if you'd like. But I'm not just sharing, I'd really like some input in advice. I'm a brand new DM. I've never even played before. I have read the DM's guide and PHB front to back, but other than that I really have no experience other than watching others play online etc. 

I have all kinds of questions and concerns about it- Is it over powered? Under powered? Is the "ramp up" too slow or too fast? Is there too many abilities and conditions to keep track of? Do any specific abilities or conditions seem out of place or un-fun? Should I re-arrange any of them? Add anything? Remove anything? Change any of them? Are there any mistakes I've made as far as game rules go? Is the gameplay/roleplay balance good? Please keep these questions in mind as your read through this homebrew, and please share your input! Thank you very much.

(please note, the formatting is a bit messed up as I copy/pasted from my design document, sorry about that)

SPELLSCARRED / PLAGUETOUCHED:

• Creatures or players who cast too much magic within a given period of time will gain the Spellscarred condition. There are different severities of the condition, and the more one uses magic while affected by Spellscarred, the more the condition advances.
•  Acquiring the condition: The Weave starts to tear around repeated spell usage.
• Casting more than 6 cantrips in a 2 hour timeframe will bestow the player with 1 level of Spellscarred after their next rest. (This is the only case where cantrips can bestow a level of spellscarred)
• Casting more than 3 level 1 spells in a 1 hour timeframe will bestow the player with 1 level of Spellscarred after their next rest.
• Casting more than 3 level 2 spells in a 1 hour time frame will bestow the player with 1 level of Spellscarred after their next rest
• Casting more than 2 level 3 spells in a 1 hour time frame will bestow the player with 1 level of Spellscarred after their next rest.
• Casting more than 1 level 4 or 5 spells in a 1 hour time frame will bestow the player with 1 level of Spellscarred after their next rest.
• Casting a level 6 spell or above will immediately grant them level 1 Spellscarred, interrupting them and causing them to go unconscious, for 3 turns or 1 hour, after the level 6 spell has been casted.
• A player can only acquire 1 level of spellscarred at a time. Example: casting 6 level 1 spells within a 1 hour period will not bestow them with level 2 spellscarred. The cooldown or acquisition resets after every long or short rest, or 24 hours.
• Mixed spell use can compound and result in 1 level of Spellscarred. Example: (2) level 1 spell, and (1) level 2 spell, Or (1) level 2 and (1) level 3. Etc; will all result in acquiring 1 level of spellscarred. 
• Particularly strong magical effects, and environmental effects, can also trigger Spellscarred acquisition regardless of level or whether or not they casted. Up to DM discretion. Example: A player reading a magical tome and being infused with power. A player spending too long in a place of power. A player being under the effect of a beneficial magical condition that affects them for extended periods of time (Longer than 1 hour). Etc.
• Curing/Treating the condition: Players must discover various cures to the Spellscarred condition, there is no commonly known cure.
• Refraining from magic use for extended periods of time will reduce the effects of Spellscarred 1 level at a time. No magic use (except from cantrips) for 48 hours, or 2 long rests, will subtract 1 level of spellscarred UP TO level 1 spellscarred. I.E: You can reduce the level of spellscarred by refraining from magic use, but level 1 spellscarred cannot be cured by refraining from magic, and must be cured via other means. However- This method no longer works after level 3 Spellscarred. (Arcana/Medicine check)
• High level magic users who have avoided the spellplague (such as War Wizards) may know of a cure or treatment..(Arcana/History check)
• Prayer to certain gods may provide them temporary relief and/or protection from the negative effects of Spellscarred. (Religion check)
• Certain magic items may remove levels of spellscarred or cure it entirely. (History check)
• Certain potions and consumables maybe reduce the effects or remove levels of spellscarred (Medicine/Nature/Alchemy check)

• Levels and Effects

• Level 1 Spellscarred: Upon waking up from your rest, you feel like shit. Your mind seems a bit cloudy, and you feel as if you didn’t get a particularly good night's rest. Upon a quick inspection of yourself, you notice that the tips of fingers have developed a faint blue hue.
• Plague Weakened: The toll this has taken on your body means you are weak to other sources of spellplague.  Take a -2 penalty to all defenses and saving throws against any other plaguechanged or spellscarred creatures or entities.
• Plague Awareness: Being spellscarred has attuned you to the Spellplague in a unique way, allowing you to be keenly aware of its presence. You can intuitively sense an area of Spellplague, Wild Magic, or plaguechanged and spellscarred creatures, within 60 feet.

• Level 2 Spellscarred: Upon waking up from your rest, you feel like shit. You have a headache, and your body is sore. You definitely didn’t get a good night’s rest. Upon a quick inspection of yourself, you notice the blue hue on your hands has intensified and spread, reaching up your arms, now a brighter hue of blue Your eyes are also bloodshot.
• The Plagues Toll: Spellcasting has begun to hurt. You feel as if the plague is feeding off your magical energy. Upon casting a level 3 or higher spell, take 1d4 force damage.
• Sight of the Unseen: The plague has begun claiming your body. Your eyes are bloodshot, but it seems as if you can see better in the dark. Grants darkvision within 10 feet

• Level 3 Spellscarred: Upon waking up from your rest, you once again feel like shit. You wake up in a cold sweat with a splitting headache, and it feels like your arms and legs are on fire. Your eyes hurt, and bright light seems particularly offensive to your eyes. Upon an inspection of your body, you see the infection of blue hued tendrils  has further spread up your arms and chest, reaching all the way up to your neck, tendrils of blue barely visible under your collar. You also notice it's begun to spread downward as well, reaching down your legs and infecting your feet.
• Whispers of the Plague: The plague has begun infecting your mind. You occasionally hear dissonant whispers. You can’t make out any particular words or meaning, but you hear them nonetheless. Often at inopportune times. It also often keeps you from getting a good night’s rest.You have a -2 to perception rolls and disadvantage on wisdom saving throws.
• Plaguespeed: As a result of the plague reaching your legs, you feel yourself capable of moving faster for short bursts, though it takes a lot out of you and strains your body. You think you could only do it a couple times a day without collapsing from exhaustion.(Bonus action) Once per encounter, gain a +1 bonus to speed or +2 if you charge or dash/run.If you moved 30’, Flamespeed now lets you move 40’ etc.
• Sight of the Unseen II: The plague has almost entirely claimed your sight. The whites of your eyes have a slight blue glow in low light, and  it seems as if you can see better in the dark. Grants darkvision within 15 feet

• Level 4 Spellscarred: You wake up in the middle of the night in a great deal of pain. Your body aches, you are sweating, and you have an excruciating headache. Upon an inspection of your body, you see the blue tendrils have made their way up across your face. Staining your visage with tendrils of a vibrant blue hue. The areas that were first infected– being your fingers, hands and arms– now have an additional purple hue seemingly layered on top of the vibrant blue.
• Plight of the Scarred: Your transformation and condition is basically impossible to hide at this point, and people begin to notice. Most don’t have favorable opinions of your condition, considering you dangerous and unstable.You have a -2 to diplomacy, persuasion, and performance checks, a -1 to charisma ability checks, and people generally regard you as untrustworthy. You do however get a +2 to intimidation checks due to your ghastly appearance.
• Plaguereach: Your hands and arms feel like they are constantly on fire, but you’ve noticed a newfound power lingering within them. You can invoke these newfound powers to bolster your spells and attacks, reaching further than they could before. The tendrils on your hands glow and pulse with a subtle blue glow when you invoke this power. It is, however, quite painful and as such you can invoke this power once a day. (Action) Once per day as a free action, your hands flare with a blue glow and you have +2 to the reach of any attack you make, including spells. 
• Plague Awareness II: Being infused with the spellplague to this extent has attuned you to the Spellplague in a unique way, allowing you to be keenly aware of its presence to an even greater effect. You can intuitively sense an area of Spellplague, Wild Magic,or plaguechanged and spellscarred creatures, within 120 feet.

• Level 5 Spellscarred: The pain in your body has grown unbearable, a constant, throbbing agony that now persists even during the day. Your skin feels as though it's being stretched and torn from the inside as the spellscar spreads further. The blue tendrils have fully enveloped your face and neck, their vivid hue now intertwined with deep, pulsating veins of purple that run down your chest and back. Your veins bulge unnaturally, and the flesh around the tendrils seems to writhe, as if alive. The first infected areas—your hands, fingers, and arms—are now mottled with dark, bruise-like patches that pulse with a faint, sickly light, shifting between hues of blue and purple..
• Fleeting Sanity: The constant pain and the progression of the Spellplague have begun to affect your mind. You experience moments of disorientation, hallucinations, or irrational thoughts, making it difficult to concentrate or trust your own perceptions. You have -2 to all Wisdom saving throws and you gain 1 level of exhaustion, giving you disadvantage on ability checks. If you already have  levels of exhaustion, this is added on top.
• Spellscarred Resilience: The twisted magic of the Spellplague has toughened your body, making you more resistant to physical harm. Your flesh, though wracked with pain, has grown unnaturally durable, and the tendrils of blue and purple energy seem to shield you from some of the worst effects of injury.You gain resistance to one type of damage (choose from fire, cold, or necrotic). Additionally, you have advantage on saving throws against spells and effects that would cause the Poisoned condition. This resilience, however, comes with the eerie sensation of your skin hardening and your nerves dulling to sensation, making you increasingly less aware of minor injuries.
• Unstable Limit Break: The magic coursing through your veins is volatile and unpredictable, but it grants you the ability to unleash powerful spells when you need them most. This instability can push your spells beyond their usual limits, though at the risk of personal harm.Once per long rest, you can choose to cast a spell at one level higher than you have spell slots for (e.g., casting a 2nd-level spell using a 1st-level spell slot). When you do, roll a d10. On a roll of 1-3, you take 1d10 necrotic damage as the unstable magic lashes back at you. This damage cannot be reduced or prevented by any means. The spell still takes effect as if cast at the higher level. This spell, empowered by the plague-energy, does not count as a spell for the purposes of acquiring another level of the Spellscarred condition.

• Level 6 Spellscarred: Your body has become a battleground, wracked with pain and twisted by the uncontrollable magic of the Spellplague. The blue tendrils now snake across your entire body, branching into intricate, glowing patterns that pulse with an unsettling, rhythmic energy. The purple veins that once accented the tendrils have darkened to a deep, almost black hue, and now cover your torso and limbs in jagged, spreading lines. Your skin feels hot to the touch, as if a fever rages beneath the surface, and your muscles twitch involuntarily. The transformation is nearly complete, and it is evident that the Spellplague’s grip on you is tightening.
• Curse of the Spellscarred: Your condition is now impossible to hide. Your appearance is not just unsettling—it is horrific. The Spellplague’s corruption has altered your voice, deepening it into a rasping whisper, and your movements have become jerky and unnatural. People instinctively recoil from you, treating you as an outcast or monster. You now suffer a -4 penalty to Diplomacy, Persuasion, and Performance checks, and a -3 to Charisma ability checks. Your appearance, however, has become so fearsome that you gain a +4 bonus to Intimidation checks.
• Spellscarred Fatigue: The constant pain and the burden of your transformation drain your strength and endurance, you are noticeably more frail than you once were. Your maximum hit points are reduced by 10.
• Plagueborn Power: The Spellplague has fully integrated into your being, granting you a measure of control over its chaotic energies. You can now channel the power of the Spellplague to enhance your spells and abilities. Once per day, you can add 1d8 force damage to any attack or spell you cast. This damage bypasses resistances and immunities. The power, however, comes with a cost—after using this ability, you must succeed on a DC 15 Constitution saving throw or gain a level of exhaustion.
• Wild Plaguestep: The Spellplague has altered your very essence, allowing you to momentarily phase out of reality, shifting through the chaotic energies that permeate the world. This gives you an uncanny ability to avoid danger and reposition yourself in battle.(Reaction) Once per short rest, when you are hit by an attack, you can use your reaction to teleport up to 15 feet to an unoccupied space you can see, causing the attack to miss you. Immediately after you teleport, roll a d6. On a roll of 1-2, the teleportation process is disorienting, and you have disadvantage on your next attack roll, ability check, or saving throw. On a roll of 3-6, you gain advantage on your next attack roll or ability check as the chaotic energy sharpens your focus.

• • Level 7 Spellscarred: Your transformation is complete. The Spellplague has consumed you, altering your very essence in ways that defy the natural order. Your body is a chaotic tapestry of vibrant blue and dark purple, the tendrils now intertwined with your veins, glowing with an unnatural light that pulses like a second heartbeat. Your skin is marred with cracks, leaking faint wisps of arcane energy, and your eyes burn with an otherworldly fire. The pain is constant, but so is the power—wild, uncontrollable, and terrifying. You have become something beyond human, a living embodiment of the Spellplague itself..
  • Cursed Exile: You are no longer seen as just dangerous—you are viewed as an abomination. People react to your presence with outright fear and hostility. mAfter using any spell, ability, or feature tied to your Spellscarred condition, you must roll a d20. On a roll of 1-9, you suffer 2d6 force damage as the chaotic energies lash out uncontrollably. Additionally, this feedback causes you to lose concentration on any active spells or effects you are maintaining, as the sudden pain and disruption make it impossible to concentrate again for 1 turn.
  • Chaotic Feedback: The unstable magic of the Spellplague periodically backfires, causing unpredictable and harmful surges of magical energy within your body. Once per long rest, you can use an action to create a 20-foot radius sphere centered on a point you can see within 60 feet. Within this sphere, you can choose one of the following effects. 
  • Chaotic Convergence: Periodically, you can feel the energies within you coalesce into a magnificent show of power, waiting to be released. Your transformation and connection to these energies gives you some semblance of control over them.
    • Support: All allied creatures within the sphere gain advantage on attack rolls, saving throws, and ability checks, and they recover 2d6 hit points at the start of their turns for 1 minute (10 rounds). Additionally, any harmful conditions affecting allies (such as stunned, poisoned, blinded) are suspended for the duration.
    • Offense: All creatures (friend and foe) within the sphere must make a DC 18 Constitution saving throw or take 5d6 force damage and have disadvantage on attack rolls, saving throws, and ability checks for 1 minute (10 rounds). Additionally, any beneficial conditions affecting enemies (such as being invisible or having temporary hit points) are suspended for the duration.
  • Plaguetouched Immortality: The Spellplague’s magic has so deeply integrated with your being that it grants you a twisted form of immortality. You can no longer die of old age, and you are immune to disease and poison. Your appearance has become so horrific that you suffer a -6 penalty to Diplomacy, Persuasion, and Performance checks, and a -4 penalty to all Charisma ability checks. Intimidation checks, however, now receive a +6 bonus, as your monstrous form strikes terror into the hearts of those who see you. If you are reduced to 0 hit points, you automatically stabilize after 1 minute as the Spellplague’s magic stitches your wounds together. However, this unnatural longevity is a double-edged sword; every time you are revived in this way, you permanently lose 1 point of Constitution as the Spellplague slowly consumes your vitality

How did this system go for you? I am planning on using spellplague in my campaign and there isn't a whole lot out there. I really like your system and am interested to hear how it worked.

This looks awesome! I say it's progression is steady and it just gets cooler (with downsides, very important). I say this seems perfect for your game. Might I ask what spell plague is though? I know quite a bit about mechanics, but official lore is a different story.

A bit late to he party but a couple of things.
1) the spellplague , and associated spells cars, began in 1385 and ended between 1485 and 1493 DR so the OP’s campaign would have been at t(e start not at the end. - and so would actually be a 4e campaign not a 5e. 5e rules start with the end of the spell plague and the return of Mystra @1485 DR.
2) while all the above info is wonderful homebrew the “official “ lore on spellscars can be found here : https://forgottenrealms.fandom.com/wiki/Spellscar

I feel like cantrips shouldn't cause spell plague because they're the "standard attack" of spellcasters. Like if a fighter could gain exhaustion from swinging his sword 6 times.