My players hit the main platform of the excavator yesterday as Hellion threw everything against them including Kulgara, Draigs and the Dark Creeper types. Dark Slayer below. I used 4e version of the Creepers as the basis for the Dark and updated them to 5e
Armour Class 16 (leather)
Hit Points 40 (8d6+16)
STR DEX CON
11 (+2) 18 (+4) 14 (+2)
INT WIS CHA
14 (+2) 14 (+2) 13 (+3)
Skills Stealth +6
Senses darkvision 60 ft., passive Perception 9
Languages Common, Elvish
Challenge 3 (700 XP)
Curse the Day. Dark ones have disadvantage in full daylight or similar affects. They also have disadvantage on saves related to blindness when a bright light is the cause.
Dark Step. The creeper can take the Disengage or Hide action as a bonus action on each of its turns.
Soul Harvest. The dark slayer deals an extra 2d6 damage on melee and ranged attacks against any target it has combat advantage against and the dark slayer gains an equal amount of temporary hit points. These temporary hit points last for a maximum of 1 hour.
Innate Spellcasting. The dark slayer innate spellcasting ability is Intelligence (spell save DC 13, +4 to hit with spell attacks). It can innately cast the following spells, requiring no components:
At will—bleed (unstableness dying creature), chill touch (1d8 necrotic, cant heal, 1r), darkness (15 ft. sphere, 4 creatures), spectral hand (allows melee range spells to be delivered at up to 60ft range)
3/day—daze monster (Wisdom save or advantage on attacks against target, 1r), death knell (you kill a dying creature and gain +1d8 hp, and advantage on spell attacks, targets have disadvantage on saves vs. your spells), inflict wounds (3d10 necrotic)
Wicked kukri. Melee Weapon Attack: +4 to hit, reach 5 ft., one target.
Hit: 6 (1d6 + 4) slashing damage.
Killing Dark. When slain, a dark slayer explodes in a spout of darkness. All creatures within 5 ft. must make DC 13 Dexterity save or become blinded.
Gear. Apart from daggers each carries an unholy symbol of Hellion (10gp)
This small humanoid is clothed in tattered rags from head to foot. Only its sinister eyes and pale hands are visible.