If I attack with a creature, opponent decides to flash-in a creature, can I cast Cyclonic Rift before that creature is assigned as a blocker, thus allow my creature through?
If your opponent casts a creature spell, then when the spell resolves the creature will enter the battlefield (C.R. 608.3), then players get priority (C.R. 117.3b). (Once you get priority, you can cast Cyclonic Rift at that time [C.R. 117.1a].) All this happens during the declare attackers step (so well before blockers are declared), and all this doesn't end the declare attackers step (which generally happens only if all players pass in a row on an empty stack) (C.R. 506.1, 508.2a; see also C.R. 500.2).
If your opponent casts a spell (including a creature with flash), and it resolves, then players get priority. (For instance, you can now cast Cyclonic Rift at this time.) All this happens during the declare attackers step, so well before blockers are declared.
Thank you. So the moment a blocker is assigned, the attacker is considered blocked, even if I cast CR at this point?
Thank you. So the moment a blocker is assigned, the attacker is considered blocked, even if I cast CR at this point?
An attacking creature that becomes blocked doesn't become unblocked merely because all its blockers are returned to their owners' hands or otherwise removed from combat (C.R. 509.1h, 506.4).
When blockers are declared in the turn based action of the declare blockers step, one of the last things happening is, that attackers that have been assigned a blocker, become blocked, and attackers that don't have any blockers assigned to them become unblocked. Removing the blokcer after that point will not change the "blocked" designation of the attacker. The attacker remains blocked until it is removed from combat or the combat phase ends, which ever comes first.
Shu Yun, the Silent Tempest WUR Voltron Control
Temmet, Vizier of Naktamun WU Unblockable Mirror Trickery
Ra's al Ghul (Sidar Kondo) and Face-Down Ninjas
Brudiclad, Token Engineer
Vaevictis (VV2) the Dire Lantern
Rona, Disciple of Gix
Tiana the Auror
Hallar
Ulrich the Politician
Zur the Rebel
Scorpion, Locust, Scarab, Egyptian Gods
O-Kagachi, Mathas, Mairsil
"Non-Tribal" Tribal Generals, Eggs
See also this thread.
Thank you. So the moment a blocker is assigned, the attacker is considered blocked, even if I cast CR at this point?
Shu Yun, the Silent Tempest WUR Voltron Control
Temmet, Vizier of Naktamun WU Unblockable Mirror Trickery
Ra's al Ghul (Sidar Kondo) and Face-Down Ninjas
Brudiclad, Token Engineer
Vaevictis (VV2) the Dire Lantern
Rona, Disciple of Gix
Tiana the Auror
Hallar
Ulrich the Politician
Zur the Rebel
Scorpion, Locust, Scarab, Egyptian Gods
O-Kagachi, Mathas, Mairsil
"Non-Tribal" Tribal Generals, Eggs
Former Rules Advisor
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Shu Yun, the Silent Tempest WUR Voltron Control
Temmet, Vizier of Naktamun WU Unblockable Mirror Trickery
Ra's al Ghul (Sidar Kondo) and Face-Down Ninjas
Brudiclad, Token Engineer
Vaevictis (VV2) the Dire Lantern
Rona, Disciple of Gix
Tiana the Auror
Hallar
Ulrich the Politician
Zur the Rebel
Scorpion, Locust, Scarab, Egyptian Gods
O-Kagachi, Mathas, Mairsil
"Non-Tribal" Tribal Generals, Eggs