When The Reaper enters, put a -1/-1 counter on each of up to two target creatures.
Whenever a creature an opponent controls with a -1/-1 counter on it dies, you may put that card onto the battlefield under your control. Do this only once each turn.
You weaken opponents' creatures with -1/-1 counters, then steal them when they die under your control. Spread -1/-1 counters across the board to kill creatures and bring the best ones to your side, grinding out value while removing threats. Close games by snowballing a stolen army or converting all those counters into wraths and aristocrat payoffs.
Repeatable creature theft turns opponents' best bombs into your assets
-1/-1 counters double as flexible removal that dodges indestructible
Jund colors give access to elite removal, ramp, and recursion
Synergizes with sacrifice and aristocrats since stolen creatures fuel value
Only one steal per turn caps the snowball speed
Relies on opponents' creatures dying with counters, so token-light or counter-immune boards blank it
The Reaper is 6 mana and theft hinges on the commander surviving
Vulnerable to graveyard-light or flicker-based metas that recur their own creatures first
Forces opponents to sacrifice, killing more creatures so The Reaper can steal them.
Add proliferate and mass -1/-1 sources like Black Sun's Zenith, Toxic Deluge, and Carnifex Demon to convert wraths into theft engines. Layer in aristocrat payoffs (Blood Artist, Zulaport Cutthroat) and free sacrifice outlets so stolen and dying creatures always generate value. Tighten the mana with fast ramp (Sol Ring, signets) and recursion (Meren, Eternal Witness) to keep The Reaper online after removal.