Blood Cultist — ruling 1

Each time a creature is put into a graveyard from the battlefield, check whether Blood Cultist had dealt any damage to it at any time during that turn. (This includes combat damage.) If so, Blood Cultist's second ability will trigger. It doesn't matter who controlled the creature or whose graveyard it was put into.

Blood Cultist — ruling 2

If Blood Cultist and a creature it dealt damage to are both put into a graveyard at the same time, Blood Cultist's ability will trigger, but it will do nothing when it resolves.

Blood Cultist : règles, fonctionnement et interactions (MTG)

Blood Cultist is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

: This creature deals 1 damage to target creature. Whenever a creature dealt damage by this creature this turn dies, put a +1/+1 counter on this creature.

How it works : Blood Cultist

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2008· Wizards of the Coast
Each time a creature is put into a graveyard from the battlefield, check whether Blood Cultist had dealt any damage to it at any time during that turn. (This includes combat damage.) If so, Blood Cultist's second ability will trigger. It doesn't matter who controlled the creature or whose graveyard it was put into.
No other card is named in this ruling.
#2October 1, 2008· Wizards of the Coast
If Blood Cultist and a creature it dealt damage to are both put into a graveyard at the same time, Blood Cultist's ability will trigger, but it will do nothing when it resolves.
No other card is named in this ruling.