Corrupter

Corruption: Any Good alignment creature or player, the corrupter has advantage rolls on, unless their class is a Paladin or Cleric. (I'm still working on it.)

Improved Critical: If this creature rolls a 19 or 20, it gains a critical hit.

Spellcasting. The Corrupter is a 4-level spellcaster. Its spellcasting ability is Wisdom. spell save DC 13, +6 to hit with spell attacks. The Corrupter has following spells prepared:
Cantrips (at will): Sapping Sting, Resistance, Green-Flame Blade
1st level (8 slots): False life, Inflict wounds, Ray of Sickness.

2nd level 6 slots: Ray of Enfeeblement, Shadow blade, Lesser healing

3rd level 4 slots: Summon undead, Vampiric touch, Elemental weapon

4th level 2 slots: Blight, Fire sheild

If anyone has any HELPFUL comments. Please tell me.

Destroyer of Life: Where ever the Corrupter walks, 5 ft. around it makes it difficult terrain. The Corrupter can walk through it without using more walk speed. 

Action Melee Attack. Quarterstaff: +6 to hit, reach 5 ft., 1 target. Hit: 6 (1d8 + 4) Bludgeoning damage. 

Action Ranged Attack. Longbow: +8 to hit, range 150/300 ft., 1 target. Hit: 5 (1d8 + 4) Piercing damage.

Restore spell slots: On a bonus action, the Corrupter can chose to restore an expended spell slot.

Watchful eye: The Corrupter can take a reaction to catch or dodge any ranged attacks and has an increased AC of +5 to Melee attacks. This has a 5 action cooldown. (One reaction)

This happens when the Corrupter's health is lower than half.

Demon Wrath (Costs 5 Actions): all creatures in sight become frightened for 3 rounds on a failed Wisdom roll of 16.

This happens when the Corrupter's health is one fourth or less.

Forceful Grab (Costs 6 Actions): The Corrupter can chose one creature 60 ft in sight and make it fall prone and 2d8 Bludgeoning damage, this counts as a bonus action when used.