Does Blowfly Infestation create a loop with Hapatra, Vizier of Poisons?
The card text is as follows
'Whenever a creature is put into a graveyard from play, if it had a -1/-1 counter on it, put a -1/-1 counter on target creature.'
When a token dies it goes to the graveyard and then disappears, so it looks like you keep generating a token and killing it. Or does the generation of the creature allow you to cast in between.
If there is a creature with 1 toughness on the battlefield when a creature with a -1/-1 counter dies, you can create an arbitrarily large number of enter the battlefield triggers as @gendolkari explains.
This is an optional infinite loop and so the game won't end in a draw because you can always choose to end it by giving Haptatra the a -1/-1 counter from blowfly infestation. If Haptara has more that 1 toughness, the loop will end because a creature does not die. If Haptara has 1 toughness, the loop will end because her ability is no longer active.
421.2. If the loop contains one or more optional actions and one player controls them all, that player chooses a number. The loop is treated as repeating that many times or until another player intervenes, whichever comes first.
This works if you start with more than one 1/1 creature.
The order of events:
When you add your first -1/-1 counter on a 1/1 creature (creature A), this triggers Hapatra’s ability.[1] When state-based actions are checked; the creature dies; which also triggers Blowfly. SBAs keep getting checked until nothing new triggers.[2] At that time, you have 2 triggered abilities to put on the stack, and you can choose the order.
No matter which order you choose, you have to choose Blowfly’s target at this time. Hapatra will not have resolved yet, so the 1/1 token won’t exist yet to target. You will have to target something else. Only after that will both abilities resolve and give you your token.
If you had a second 1/1 token (creature B) to start with, then you could target that. If you do, and you have Hapatra resolve first, then Hapatra will create another 1/1 token (creature C), followed by Blowfly adding a -1/-1 token to creature B. This then repeats the entire cycle; everything is the same now except you have creatures B and C, with B dying, instead of creatures A and B, with A dying.
Note that you can always break the loop at any time by targeting a creature with more than 1 toughness with Blowfly's trigger. Because you have a different choice you can make, the game will not end in a draw due to this loop.[3]
[1]603.2. Whenever a game event or game state matches a triggered ability’s trigger event, that ability automatically triggers. The ability doesn’t do anything at this point.
[2]704.3. Whenever a player would get priority (see rule 116, “Timing and Priority”), the game checks for any of the listed conditions for state-based actions, then performs all applicable state-based actions simultaneously as a single event. If any state-based actions are performed as a result of a check, the check is repeated.
[3]104.4b [..] Loops that contain an optional action don’t result in a draw.
