And just look at all the Knights in the format: Benalish Knight, Burning Shield Askari, Cadaverous Knight, Fallen Askari, Femeref Knight, Knight of Valor, Searing Spear Askari, Shadow Rider, Southern Paladin, Suq'Ata Lancer, Zhalfirin Commander, Zhalfirin Crusader, and Zhalfirin Knight
You could do something as simple as adding an extra -1/-1 counter to an enemy creature, or you could really ruin someone's day by doling out poison counters at an alarming rate
Naturally, Games Workshop wasn't gonna let Wizards play with its toys unless they laid down the lore first, and Warhammer Community assures the crossover "involved substantial input from the Warhammer 40,000 design studio, ensuring every last detail matched the Warhammer hobby you know and love
Whenever a creature is put into your graveyard from the battlefield, return it to your hand