So what I'am wondering is if I have Vampiric Sliver (All Sliver creatures have "Whenever a creature dealt damage by this creature this turn dies, put a +1/+1 counter on this creature.") and Bonescythe Sliver (Sliver creatures you control have double strike).
When do I add the +1/+1? is it after the 2nd round of damage is over or as soon as the 1st set of damage is done? assuming it kills something.
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-MadMage
The +1/+1 counter is put on when the ability granted by Vampiric Sliver resolves (not necessarily at the exact moment when the Sliver in question deals damage or the creature dealt damage by it dies) (see also C.R. 603.2).
If the Sliver in question has double strike and deals combat damage to a creature during the first combat damage step, and the creature dealt damage this way dies, the ability in question will trigger. When all players pass and the ability resolves (which will happen during the same combat damage step), and the Sliver in question is still on the battlefield, a +1/+1 counter is put on that Sliver. Note that if the Sliver was also dealt combat damage itself, it has to survive the combat damage in order to get a +1/+1 counter put on (see also the reminder text for Siegehorn Ceratops).
Remember that all double strike does is give a combat phase two combat damage steps rather than one (and restrict which creatures assign combat damage during those steps) (C.R. 702.4b). Double strike doesn't mean—
that a creature with that ability assigns twice its power in combat damage (see also this thread), or
that that creature assigns and deals combat damage, then assigns and deals combat damage again, with nothing in between.
When do I add the +1/+1? is it after the 2nd round of damage is over or as soon as the 1st set of damage is done? assuming it kills something.
Using card tags is mandatory in the Magic Rulings forum. Please check your private messages to learn how to use them. I've added them for this post.
-MadMage
If the Sliver in question has double strike and deals combat damage to a creature during the first combat damage step, and the creature dealt damage this way dies, the ability in question will trigger. When all players pass and the ability resolves (which will happen during the same combat damage step), and the Sliver in question is still on the battlefield, a +1/+1 counter is put on that Sliver. Note that if the Sliver was also dealt combat damage itself, it has to survive the combat damage in order to get a +1/+1 counter put on (see also the reminder text for Siegehorn Ceratops).
Remember that all double strike does is give a combat phase two combat damage steps rather than one (and restrict which creatures assign combat damage during those steps) (C.R. 702.4b). Double strike doesn't mean—
See also this thread and this thread.
Bonescythe Sliver
As soon as a creature is killed by a sliver, whenever that happens.
(allowing a short delay, while the ability spends time the stack)
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules