Merciless Human

A subclass hoping to capture the themes of Chara, from Undertale. Might befit characters who fear a world of dangerous uncertainty, preferring the comfort of known statistics and values less than their defense.

"My best friend's favorite number is nine.
It's because there isn't a number that's higher. 
9. 99. 999. 9999.
If everything gets high enough, you become invincible.
Nothing can hurt you anymore.
Nothing can hurt anyone anymore."

Level 3: Your Lucky Number

At 3rd level, you have a special love for the largest digit, and it appears to aid you from time to time; When you roll a 6 on any die, treat it as a 9 instead. This includes damage dice, and if it occurs in such a case, you can roll an additional d6 and add it to the total damage.

Level 3: Kill or Be Killed

Also at 3rd level, your gaze holds a dreadful resignation that tells your foes with certainty: This is the way things are in this world. Whenever a creature meets your eyes, you can choose to make them the target of this feature for the next minute (no action required). While under its effects, the target cannot willingly move further away from you, and they can only take actions that deal damage to you. Attempting to take any other action causes them to instead become Stunned until the start of their next turn.

You can use this feature a number of times equal to your proficiency bonus, and you regain all expended uses when you finish a short or long rest. 

Level 7: Becoming Stronger

At 7th level, you value the strength of a single coldhearted strike to pierce the defense of anything in your way. Whenever you take the Attack action and have full movement, you can spend all of your movement to prepare a terrible blow. If you do so, rather than making multiple attacks you instead make one attack that gains an additional +3 bonus to hit for each attack that you would otherwise have been able to make. On a hit, it inherits the damage of those attacks - for instance, if you could attack twice with a spear and instead chose to use this feature, the attack would deal 2d6 piercing damage + twice your Strength modifier.

Additionally, if this attack reduces an enemy to 0 hit points, the target disintegrates into dust and drops all of their personal belongings - even belongings that they weren’t wearing or carrying, which are somehow summoned to spill out from among the ashes as a grim reward. Plot-relevant artifacts might be excluded from this effect at the DM’s discretion.

Level 10: HP. GOLD. EXP. LV.

At 10th level, it's easy for you to gauge who you could kill if you had to. You gain a special sense that allows you to magically view the name, hit points, challenge rating, carried gp, and creature type of any creature in your line of sight, perceiving a metric of quantified strength that makes sense only to you.

Additionally, if at any point you make an ability check contested by another creature’s ability check or DC, and the modifier added to your roll is higher than the modifier added to their roll or DC, then you automatically succeed.

Level 15: Determination

At 15th level, your certainty keeps you moving even when your body can't anymore. While Kill or Be Killed’s duration lasts on at least one target, you cannot become unconscious or incapacitated even if reduced to 0 hit points. If it ends while you are at 0 hit points (such as because the target died), you immediately fall unconscious.

Furthermore, you’re done pretending to be human; you no longer need to eat, drink, or breathe, and your creature type is Fiend.

Level 18: When People Call Its Name

It doesn't matter when. It doesn't matter where. At 18th level, you know immediately when anyone in any plane of existence says your name, and when they do so, you gain a vague awareness of the circumstances under which it was spoken. This includes the details that your "HP. GOLD. EXP. LV." feature can reveal pertaining to the speaker and any creatures within 60 feet of them regardless of line of sight. 

Additionally, once per long rest you can use your reaction when this occurs to teleport to an unoccupied space within 5 feet of the speaker. For three rounds thereafter, you can use any limited-use feature or spell of yours without expending the normal resource (such as using Becoming Stronger without expending movement), as you find it suddenly very easy to bring yourself to hurt others.