If I react to an ability "When you play me" by playing Rengar on the other side, beginning a showdown, does the "When you play me" ability of my opponent resolve before showdown begins or does the chain wait for the showdown to end?
The "When you play me" resolves first, during the current Chain. The Showdown happens afterward.
Here's why:
-
Opponent's card is played. If it's a unit, it enters the board at finalization (Rule 337.1.c). The "When you play me" (a Play Effect per Rule 383.4.a) triggers immediately after — becoming a Pending Item on the Chain.
-
Reaction window opens. After the trigger finalizes, Priority goes to the opponent (controller of the newest item). If they pass, you gain Priority and play Rengar, Pouncing as a Reaction. Rengar is finalized and enters the battlefield — this applies Contested, but the Chain is still resolving so no Cleanup/Showdown happens yet (Rule 321).
-
The Chain resolves normally (LIFO). Any items added after the "When you play me" trigger resolve first. But once the Chain empties, the "When you play me" — which was already on the Chain from before Rengar entered — resolves. The Showdown is not inserted into the Chain.
-
Only after the Chain fully empties does the Outstanding Cleanup occur (Rules 319.6, 321.1). That Cleanup stages a Combat at the now-contested battlefield (Rule 323.9), and the Showdown begins from a Neutral Open State (Rules 323.11–323.13).
In short: the Chain finishes first — including your opponent's "When you play me" — and then the Showdown starts. You can't interrupt a resolving Chain with a Showdown; the Showdown waits for Open State.