For effects that go on the stack (like a sorcery such as Hell to Pay, or an instant such as Ram Through), it's pretty simple: Whichever effect delivered the (over)killing blow, that's the source that dealt lethal damage
This card enters unattached and stays on the battlefield if the creature leaves.) Slith Ascendant Rarity: Common TYPE: Creature SUBTYPE: Slith CMC: 3 Mana cost: USD: $0.09 EUR: 0.07 TIX: 0.03 EDHREC RANK: 24522 (1/1) Flying Whenever this creature deals combat damage to a player, put a +1/+1 counter on it
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Bloodchief Ascension's first ability has an "intervening 'if' clause." It won't trigger at all unless an opponent has already lost 2 or more life by the time the end step begins
Both the angels and demon work with the graveyard: Reya resurrects creatures from my grave, Deathpact resurrects herself from my graveyard, and when an opponent's graveyard gets filled, Kothophed makes me lose a life but gain card advantage