ジェスカイの長老

果敢（あなたがクリーチャーでない呪文を１つ唱えるたび、このクリーチャーはターン終了時まで＋１/＋１の修整を受ける。）
ジェスカイの長老がプレイヤー１人に戦闘ダメージを与えるたび、あなたはカードを１枚引いてもよい。そうしたなら、カードを１枚捨てる。

Jeskai Elder

Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)
Whenever this creature deals combat damage to a player, you may draw a card. If you do, discard a card.

Double Masters 2022

Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)
Whenever Jeskai Elder deals combat damage to a player, you may draw a card. If you do, discard a card.

Core Set 2021

Khans of Tarkir

Duel Decks: Speed vs. Cunning

Any spell you cast that doesn't have the type creature will cause prowess to trigger. If a spell has multiple types, and one of those types is creature (such as an artifact creature), casting it won't cause prowess to trigger. Playing a land also won't cause prowess to trigger.

Once it triggers, prowess isn't connected to the spell that caused it to trigger. If that spell is countered, prowess will still resolve.

Prowess goes on the stack on top of the spell that caused it to trigger. It will resolve before that spell.

You draw a card and discard a card all while Jeskai Elder's ability is resolving. Nothing can happen between the two, and no player may choose to take actions.