I've been itching to write a fast assassin for a while, one that uses unusual mechanics.
Assassins tend to be very peculiar to a particular campaign, so I'm going with a popular name for this nasty fellow: Mack the Knife.
I want to do two things with this monster: make him deadly against his foe, and incredibly fast.
For stats, I think the Shield Guardian is a good starting point: a CR 7 creature with a few abilities but focuses on simple combat attacks.
We'll swap out the Shield Guardian's bound, regeneration, and spell storing abilities, as well as its immunities, with the ability to choose his position in initiative every round and advantage on attacks against one target per long rest.
Imagine that: an assassin who simply decides when to sidle up and slip the knife between your ribs, and who will almost certainly be able to do so.
OK, let's do this!
Homebrewery stat block:
Mack the Knife
medium humanoid, evil
- Armor Class 17
- Hit Points 140 (15d10 + 58)
- Speed 30 ft.
STR DEX CON INT WIS CHA 14 (+2) 14 (+2) 12 (+1) 7 (-2) 10 (+2) 8 (-1)
- Skills Intimidation +6, Investigation +4
- Senses Passive Perception 12
- Languages Common
- Challenge 7 (2,900 XP)
Faster Than You. Instead of rolling for initiative, Mack can take his turn at any point in the initiative order, and can change this every round.
Targeted Killing. Once per long rest, Mack chooses a favored target. Mack has advantage on attacks against this target.
Multiattack. Mack makes two melee attacks or two ranged attacks.
Assassin's Dagger. +7 to hit, reach 5 ft., one target. Hit: 7 (1d6+4) piercing damage plus 3 (1d6) poison damage.
Hand Crossbow. +7 to hit, range 30/120, one target. Hit: 11 (2d6+4) piercing damage.