If my opponent casts Challenge on Abandoned Hall, does he get the +1 before resolution or after resolution?

According to RiftJudge FAQ #10497 and RiftJudge FAQ #10217, your opponent does not receive the +1 Might from Abandoned Hall before the spell resolves.

The trigger on Abandoned Hall, "When a player plays a spell," occurs only after the spell has successfully completed its resolution process.

Sequence of Events:

1. Your opponent plays Challenge and targets two units. Challenge is placed on the chain.
2. Players have the opportunity to play reactions to modify Might or the state of the board.
3. Challenge resolves: the units deal damage to each other based on their current Might values at that moment. (Because Challenge does not snapshot Might, any reactions played in step 2 will affect the damage dealt).
4. After Challenge finishes resolving, the "When a player plays a spell" condition of Abandoned Hall is met, and the triggered ability is placed on the chain.
5. Your opponent then chooses a unit to receive +1 Might.

Note: If the units died during the resolution of Challenge, they cannot receive the +1 Might buff from the hall because they are no longer on the board to be targeted.