Does using the spell Challenge count as combat?

No, using Challenge does not count as combat.

Challenge is a spell — it causes two units to deal damage to each other equal to their Might, but this is a non-combat interaction (FAQ #11387, #8835). Combat in Riftbound is a specific game phase that only occurs when a battlefield is Contested and units controlled by opposing players are present, triggering the structured Showdown → Combat Damage → Combat Cleanup sequence (Rules 454–458). Challenge bypasses all of that entirely.

This matters for a few things: a stunned unit can deal damage through Challenge (stun only blocks combat damage, FAQ #3841), Challenge won't trigger "when I win a combat" effects like Draven, Audacious (FAQ #8835), and the damage dealt persists on those units until either any combat resolves (which heals all units) or the turn ends (FAQ #7781).