If you control both Niv-Mizzet, Parun and Niv-Mizzet, Visionary, their triggered abilities can form a chain: Parun’s “Whenever a player casts an instant or sorcery, you draw a card” trigger draws you a card, which creates one or more “Whenever you draw a card, Niv‑Mizzet deals 1 damage to any target” triggers from Parun. Each of those damage resolutions that deals noncombat damage to an opponent will trigger Visionary’s “Whenever a source you control deals noncombat damage to an opponent, you draw that many cards.” Those extra draws then re-trigger Parun’s draw-triggered damage, and so on. Players get priority and a chance to act between triggered abilities resolving as described in and ordering for multiple players is APNAP per . Both creatures can coexist because they have different names (the legend rule in does not force one off).
Step 1
You cast an instant or sorcery. Niv‑Mizzet, Parun’s triggered ability “Whenever a player casts an instant or sorcery spell, you draw a card” triggers and is put on the stack.
Step 2
That triggered ability resolves (see ). You draw a card. The draw event causes Parun’s triggered ability “Whenever you draw a card, Niv‑Mizzet deals 1 damage to any target” to trigger (once per card drawn) and those triggers are put on the stack.
Step 3
One of Parun’s draw-triggered abilities resolves (each resolves separately). As it resolves it deals 1 damage to the chosen target (the instruction on the ability is followed in order per ). If that damage is dealt to an opponent as noncombat damage, Niv‑Mizzet, Visionary’s ability “Whenever a source you control deals noncombat damage to an opponent, you draw that many cards” triggers for that damage and is put on the stack (the source of the damage is the Niv‑Mizzet that dealt it per and ).
Step 4
Visionary’s draw-trigger resolves and you draw cards equal to the damage dealt. Each of those draws causes Parun’s draw-trigger to trigger again (one Parun damage trigger per card drawn), creating more damage triggers on the stack.
Step 5
Repeat resolution of Parun damage triggers → Visionary draw triggers → Parun draw triggers until no further triggers are created or an intervening condition stops the chain (for example, if a Parun damage trigger has no legal targets and is countered under , or if the game ends because of drawing from an empty library under and ).
Step 6
Between each resolution of abilities, players receive priority and may cast spells or activate abilities (see about resolution and priority).
Resolution
Casting an instant or sorcery while you control both Niv‑Mizzet, Parun and Niv‑Mizzet, Visionary can start a loop of draws → Parun damage → Visionary draws → Parun damage, etc. Each event is a triggered ability that’s put on the stack and resolves one at a time following the rules for resolving spells and abilities and for handling triggered abilities . Players get priority between resolutions so they can cast spells or activate abilities between those triggers , and multiple triggered abilities are ordered APNAP when necessary . The loop will stop if a required action can’t be performed (for example, there are no legal targets for Parun’s damage and that damage trigger is fully illegal and therefore countered under ) or if it runs into game-ending consequences such as drawing from an empty library (causing a loss under with drawing details in ).
