#[Undertale spoilers!]
... Yup. I just finally finished it (sorta - I'm not a monster, you know) almost two years after its release.
So!
##Asriel Dreemurr, God of Hyperdeath
(Melee mage/fighter)
Passive: SOUL Drain & Chaos Sabre
SOUL Drain: Whenever Asriel lands the final blow on an enemy champion human, monster, or epic boss monster, he absorbs their SOUL. Each monster SOUL Asriel absorbs grants him 1 AP. Each Durable SOUL (Human or Boss Monster SOUL) Asriel gains increases this bonus by 10%, stacking additively. However, once Asriel has absorbed 7 Human SOULs, the bonus from all human SOULs instead increases to 100%, and remains there. Boss Monster SOULs continue to stack at a rate of 10% per SOUL.
Chaos Sabre: Every 60-20 seconds (scaling by level), Asriel charges a Chaos Sabre, and can hold a number up to 1 + the current number of Durable SOULs he possesses. While Chaos Sabre is available, Asriel's basic attack range is doubled, and each attack fires a Chaos Sabre in a line [moderate speed short range skill shot] that deals damage based on Asriel's Ability Power.
Once he obtains 7 Human SOULs, this passive becomes Chaos Slicer, increasing its damage by 50% and halving the charge cooldown.
Q: Fire Magic
Asriel fires a single ball of fire, dealing moderate magic damage [moderate base, moderate scaling] to the first enemy it hits. Very short cooldown, low mana cost.
When Asriel obtains 7 Human SOULs, Asriel can immediately recast this ability twice with an 0.1s lockout duration and no additional cost. Cooldown begins on first cast.
W: Shocker Breaker/Chaos Buster
Passive: Shocker Breaker
Every x seconds, a random location near Asriel is struck by a bolt of lightning [telegraphed briefly], dealing minor magic damage [low base, high AP scaling].
When Asriel obtains 7 Human SOULs, this becomes Shocker Breaker II, and fires 3 additional smaller bolts (1/3rd damage) at random nearby locations in addition to the primary bolt each time it procs.
Active: Chaos Buster
Asriel fires 9 lines of 10 stars stars in a set pattern, dealing minor magic damage [low base, low AP scaling] per star. If at least 1 star hits, this ability becomes Chaos Blaster, and Asriel can reactivate it to fire a large beam that deals high magic damage [high base, high AP scaling].
When Asriel obtains 7 Human SOULs, Chaos Blaster also fires a small point-blank AoE around Asriel, dealing extreme magic damage [double the damage of Hyper Blaster] in a range of 150 [edge-to-edge] around him.
E: Star Blazing
Asriel fires a single star-shaped projectile that explodes upon hitting an enemy or reaching maximum range, dealing moderate magic damage [high base, low AP scaling]. When the projectile explodes, it fires smaller stars in all directions, each of which deals half damage to any target they hit. Each enemy can only be damaged once by this attack.
When Asriel reaches 7 Human SOULs, this ability becomes Galactica Blazing, and instead fires 3 stars. Each enemy can only be damaged once by any large star.+
R: Hyper Goner
Asriel summons a giant skull-like object at the target location, which immediately begins to drag all nearby enemies and non-epic monsters towards it at a speed of 400/second. Non-champions are instantly destroyed if they are dragged into its mouth. Champions take extreme magic damage [low base, extreme AP scaling]. This ability cannot kill enemy champions, and leaves them at 1 HP if they would have otherwise died.
Once Asriel has absorbed 7 Human SOULs, he can recast this ability to become the Angel of Death, immediately gaining 50% of his total AP as bonus AD and granting him +30% movement speed [multiplicative] for x seconds. All of his abilities are changed to Comet for the duration. Comet fires 2 homing Fire Magic projectiles [can be a skill shot, which will home in on the nearest enemy, or a targeted ability, which will home in on the target] on a 1s cooldown.
[based completely off of the Undertale Wiki, because seriously would I actually remember all of these ridiculous names?]
Alternatively, the final boss could be Chara. That seems like it'd be fun to build, but I don't have time at the moment. Maybe later!