Servant of Slaughter

In terms of flavour, this subclass functions far more like a Warlock subclass (and making this for Warlock was the original intent), however, for a subclass so inherently connected to combat, using a non martial class felt wrong.

When picking this subclass, your powers are drawn from a patron. No rules for what the patron is, but a deity or eldritch entity focussed on war and carnage feels most appropriate. Could be a demon or devil as well, whatever works for your setting. In selecting this subclass, you also sacrifice Second Wind (not a huge concern as you will acquire healing options later.

This is a non-magic subclass, with no spell casting. The core principal of this subclass is exactly what it says on the tin, empowering yourself through combat so you can remain in combat for longer. This is also not melee specific. Thematically it works better in melee, but you won't be penalised for fighting from range.

The Servant of Slaughter is built around being an unstoppable force in battle, killing more to get stronger, and getting stronger to kill more. Ideally, it should be something of a building wave, something that must be stopped before it gets started, or it becomes almost impossible to stop. This is balanced by the fact that this subclass gains next to nothing outside of combat, and, once combat ends, must build up that momentum all over again.

Also, this should go without saying, but, this is homebrew. If your DM doesn't allow it, just accept it. Once you become able to reliably land hits, this subclass becomes extremely powerful. I would only allow it at my table because I am a very harsh DM, and there is a non-zero chance that one or more players will die every session. But at your table, it's your DM's decision, not mine, so if they feel that it will unbalance the campaign, just agree and move on. Honestly this feels more like the subclass you might give to a BBEG.

Oh, and in terms of multi-classing, there shouldn't be any mechanical issues (except with Barbarian I guess, but... why?), the only issues I can see being purely in terms of flavour, with Cleric, Paladin, Warlock and maaaaybe Druid being the only classes where I can't see it working (considering that their deities/patrons are unlikely to appreciate double-dipping). Oh and Monks. There theoretically is nothing preventing you from multi-classing into Monk, it just doesn't feel right, y'know?

And a final note, this works really well with a custom rule of mine called Martial Prowess, where every martial class gains Expertise (Rogue-style) in a weapon proficiency of their choosing (specific weapon not weapon category), with Fighters getting 2. Anyway I hope you like it.

Bloodpoints

At 3rd Level, Servants of Slaughter gain access to Bloodpoints. Whenever the player successfully deals damage with an attack roll, they acquire 1 Bloodpoint. These points can then be spent to turn the tide of battle. There is no maximum number of Bloodpoints a player can accumulate. Bloodpoints are only generated by dealing damage with an attack, and do not generate outside of combat. Any Bloodpoints not spent within a few minutes of combat ending are lost, resetting the player's total to 0.

Bloodpoints can be spent on the following:

• Empowered Strikes: As a free action, spend any number of Bloodpoints to empower an attack. Every Bloodpoint spent grants an additional 1D6 damage, of the same type as the weapon wielded, to the attack. Attacks must be empowered before the attack roll is made, and cannot be spent retroactively. Should an empowered attack miss, all expended Bloodpoints are lost.
• Strength from Slaughter: As a bonus action, spend any number of Bloodpoints to regain strength and stamina. For every Bloodpoint spent, heal 1D6 damage. Does not remove negative status effects.
• Mark for Death: As a bonus action, spend any number of Bloodpoints to mark a target creature. Every Bloodpoint spent grants an additional round's duration during which all of your attacks against the marked creature are made at advantage. E.G. 1 Bloodpoint grants 1 round of advantage, Bloodpoints grants 2 rounds of advantage, etc.
• Relentless Assault: As a free action, spend 2 Bloodpoints to make an extra attack action. There is no limiting factor on the number of times this ability can be used, other than the number of Bloodpoints you have accumulated.

In addition starting at 15th level, upon Bloodpoint generation, an additional point is added to a separate pool. The points in this pool cannot be spent until the player lands a critical hit, at which point all Bloodpoints in the pool must be spent to empower the attack, at which point the pool is reduced to 0. Any remaining points are lost at the end of combat.

Eyes of The Carrion Lord

The methods with which such entities aid their servants are as varied as they are. Some might take the form of a flock of crows, or as circling vultures. Others may watch through tattoos, through a missing eye, or by instilling a part of themselves into their servant's weapon, allowing it to move on its own. Regardless, at this stage the survival of their servant has become of interest to them, and so at 7th Level, they grant these followers a fragment of their vast, even omniscient awareness, providing immunity to being Flanked or Surprised, as well as advantage on Wisdom (Perception) checks. 

Also, at 10th level, their servant's survival becomes of genuine importance to the player's patron, granting them a +2 bonus to AC.

Wrath and Ruin

Starting at 10th level, a Servant of Slaughter can expend 7 Bloodpoints to enter a trancelike rage. Unlike the frothing rage of a Barbarian, a Servant of Slaughter's rage provides no offensive bonuses. Instead, the joy of the bloodbath around them allows Servants of Slaughter to ignore a significant amount of pain, and the otherworldly energies of their patron, flowing through their veins, allow them to shrug off even very powerful hits. As such, upon entering this rage, they gain Resistance to all forms of Slashing, Piercing and Bludgeoning damage. This rage only ends after 1 minute, or if they neither deal nor receive any damage for the duration of 1 round.

In addition, at 15th level, a Servant of Slaughter is now among the foremost champions of their patron. Therefore, as a possible future avatar of their patron, their survival is of utmost importance. At this level, enraged Servants of Slaughter gain resistance to all types of damage for the duration of their rage.

Savage Strength

Starting at 15th level, whenever a Servant of Slaughter kills another creature, Savage Strength activates. With Savage Strength active, the next attack the player makes is made as normal, however, if the attack lands successfully, it is considered to have critically hit, and damage is dealt as such. After that first attack is made, Savage Strength deactivates until the next time the player scores a kill.

Battle Made Manifest

Starting at 18th Level Servants of Slaughter become the avatars of their patron. At this level, they gain the following benefits.

• An additional 1D6 of damage to any weapon attack, provided they have accumulated at least 1 Bloodpoint.
• As a bonus action they may deal 1D6 Necrotic damage to themselves to ensure they begin every encounter with 1 Bloodpoint.
• Every time they claim 25 kills, they may choose to spend 10 Bloodpoints to increase one ability score out of Strength, Constitution and Dexterity by 1, with the maximum for these ability scores increasing to 26. 

This capstone level for the Servant of Slaughter may not feel hugely impactful, as it is a lot of little things. However, those things can completely change, the flow of an encounter. As for the kill requirement, the number I settled on was somewhat arbitrary. Thematically, it could be significantly larger, but hordes of chaff aren't that common in high level encounters. Therefore 25 seemed like an appropriate middle ground, to make the improvements rare, but not unattainable over the course of a few sessions. However, if you as a DM run high level combat with a few, high level enemies (as is often the case), or if you Servant of Slaughter player has difficulty getting to enemies in order to kill them, or if you are running a low combat campaign (in which case I question having a Servant of Slaughter in your party at all), then absolutely feel free to lower the kill requirement, to make a bit more out of this feature. Similarly, if you run a lot of battles, or like to fill combat encounters with spongey enemies sticking to the 25 kill requirement could leave you with a Servant of Slaughter who gains multiple stat improvements per encounter, so, again feel free to increase the requirement as necessary.