
Whenever Grim Reaper attacks, you may pay {3}{B}. When you do, return target creature card from your graveyard to the battlefield tapped and attacking with a finality counter on it. (If a creature with a finality counter on it would die, exile it instead.)
How do you want this deck to play?
What the commander does best.
Attack, pay four, and drag your biggest dead creature back into the fight, tapped and swinging.
340 cards fit this theme
Gather a rogues' gallery of Villains and legendary menaces, then let Reaper bring the fallen ones back.
70 cards fit this theme
Extra combats and attack triggers give Reaper more swings and more chances to raise the dead each turn.
294 cards fit this theme
Off the beaten path, but they work.
Sacrifice your own creatures for drain value, then reap them back as attackers with finality counters.
399 cards fit this theme
Mill and surveil yourself into a huge graveyard, then choose the best corpse to bring back.
203 cards fit this theme
Reanimate creatures with enter-the-battlefield triggers and blink them for repeated value every attack.
516 cards fit this theme