With Fire Lord Azula attacking, casting Frantic Search and Narset's Reversal can produce an effectively infinite loop because Narset's Reversal can copy and return Frantic Search to your hand while Frantic Search (or copies of it) untap lands that let you recast spells; Azula's triggered ability will copy each spell you cast (putting additional copies on the stack) rather than causing those copies to be cast .
Step 1
You cast Frantic Search (it becomes a spell on the stack; this is a cast event under rule ).
Step 2
Fire Lord Azula sees a spell was cast while she is attacking and a triggered ability is put on the stack to copy that spell later (Azula’s ability triggers on casting).
Step 3
While the original Frantic Search is still on the stack, you cast Narset's Reversal targeting that Frantic Search (casting Narset's Reversal is a separate cast event and Azula will trigger again for that cast) .
Step 4
Azula's triggered ability(s) are now on the stack above Narset's Reversal (the trigger(s) were put on the stack when the spells were cast) .
Step 5
Let the top Azula trigger resolve first: it creates a copy of Narset's Reversal on the stack. That copy is put onto the stack and is not cast; you may choose new targets for that copy as allowed by the copy effect (Ruling [2019-05-03]).
Step 6
That copy of Narset's Reversal can target the original Frantic Search that is still on the stack (targets are chosen when the copy is created) .
Step 7
When the copy of Narset's Reversal resolves it copies the targeted Frantic Search and then returns the original Frantic Search to your hand; the copy of Frantic Search is created on the stack (it is not cast) (Ruling [2019-05-03]).
Step 8
The copy of Frantic Search resolves, drawing two cards and discarding two cards as a single uninterrupted resolution and untapping up to three lands (you choose which lands to untap as it resolves) (Ruling [2018-12-07]) (Ruling [2022-12-08]).
Step 9
Because the original Frantic Search was returned to your hand by Narset's Reversal's effect, you now have Frantic Search back in hand and additional untapped lands that you can use to cast more spells (including Frantic Search and Narset's Reversal) again, repeating the sequence under the assumed resource conditions.
Step 10
Note that copies placed on the stack (the Frantic Search copies and the Narset's Reversal copies) are not casts, so Azula's ability does not trigger from those copies; Azula only triggered for the spells you actually cast (Ruling [2019-05-03]).
Resolution
When you cast Frantic Search while Fire Lord Azula is attacking, Azula's triggered ability will trigger and later create a copy of the Frantic Search spell on the stack; copies are put onto the stack and are not cast (so they do not themselves cause Azula to trigger) (see Ruling [2019-05-03]). If you cast Narset's Reversal targeting Frantic Search on the stack, Narset's Reversal will copy that Frantic Search and return the original to your hand; the copy of Frantic Search resolves and untaps lands (draw two, discard two happen together during that resolution) (Ruling [2018-12-07]) (Ruling [2019-05-03]). Using the untapped lands you can recast Frantic Search and Narset's Reversal again, repeating the process; because copies are put on the stack rather than cast, Azula only triggers for the spells you actually cast, and Narset's Reversal gives you a way to return Frantic Search to your hand while producing a resolving copy that untaps lands, enabling the loop under the assumed resource conditions (see rules and rulings below) (Ruling [2019-05-03]) .
