If I play Challenge with a unit in Abandoned Hall, will the unit gain the +1 Might from Abandoned Hall before fighting?

Ruling: No. The unit will not receive the +1 Might from Abandoned Hall before Challenge resolves. Abandoned Hall's "when you play a spell" trigger occurs after the spell has completed its resolution, not when it is placed on the chain.

Sequence:

• You play Challenge, declaring targets (your unit and the opponent's unit). Challenge goes on the chain.
• Both players have a reaction window to play reaction-speed cards that could modify Might.
• Challenge resolves using the units' current Might values at resolution time (Challenge does not snapshot Might).
• After Challenge finishes resolving, Abandoned Hall's trigger is placed on the chain.
• If your unit died during Challenge's resolution, it cannot receive the +1 Might buff.

Nuances:

• Challenge does not snapshot Might values. Reaction-speed effects played before Challenge resolves will affect the damage dealt.
• "When you play a spell" triggers occur after the spell completes its resolution process.