If I have a unit at Abandon Hall. I play Challenge on my unit and the opponent unit. Does Abandon Hall give +1 might before or after Challenge?

According to RiftJudge FAQ #10497 and RiftJudge FAQ #10217, Abandoned Hall will give your unit +1 Might after Challenge has finished resolving, not before.

Sequence of Events:

1. Play Spell: You play Challenge, targeting your unit and the opponent's unit. Challenge is placed on the chain.
2. Reaction Window: Both players have an opportunity to play [Reaction] cards to modify Might or the state of the board.
3. Resolution: Challenge resolves. Your unit and the opponent's unit 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. Trigger Condition Met: Once Challenge has successfully completed its resolution process, the "When a player plays a spell" condition on Abandoned Hall is fulfilled.
5. Trigger Placed: The Abandoned Hall triggered ability is placed on the chain.
6. Resolution: You resolve the Abandoned Hall trigger and choose your unit to receive the +1 Might buff.

Note: If your unit is destroyed during the resolution of Challenge, it is no longer on the board and therefore cannot be targeted to receive the +1 Might buff from the Abandoned Hall trigger.