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.
07-08-2020
wotc
You draw one card when the ability resolves, not one card per creature that died during the turn.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.