If a creature didn't die before a turn's end step begins, Deathreap Ritual's ability doesn't trigger at all. The creature may have died before Deathreap Ritual entered the battlefield, however.
2020-08-07
You draw one card when the ability resolves, not one card per creature that died during the turn.
Morbid — At the beginning of each end step, if a creature died this turn, you may draw a card.
