I was playing with someone running the Jeska, Thrice Reborn and Falthis, Shadowcat Familiar partner combo. They explained that if they have Jeska in play and then cast Falthis, they can use Jeska to deal deathtouch damage to three creatures. The part I don't understand is that there's apparently a difference between removing some of the counters from Jeska and removing all of them. If only some of the counters are removed to the -X, you can save the targeted creatures with a removal spell on Falthis in response. But if all the counters are removed for the -X, you get no opportunity to respond and the deathtouch damage is guaranteed. This was explained to me that Jeska can only lose deathtouch if it's in play, not in the graveyard. I don't understand this, why can't I respond to Jeska's -X with a Lightning Bolt on Falthis to stop the deathtouch no matter how many counters are taken off?
When a source of damage leaves the zone it is expected to be in (usually the battlefield) before it deals damage, the game uses last known information (LKI) about the source, looking for anything relevant in its characteristics to the damage dealing event. Deathtouch and lifelink, wither and infect are all characteristics that influence damage, and LKI has Jeska having had deathtouch. This doesn't change if Falthis leaves the battlefield, too, because Jeska is already gone, her LKI doesn't change anymore.
Former Rules Advisor
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