Hi all, we have a PC that just found a Horn of Valhalla alongside some other dwarven treasures in a dragon horde.  We are flavoring it as the spirits are dwarven battleragers that have fallen.  The written language on the Horn is a little vague though when it comes to combat.  Do the spirits summoned attack on the turn they are summoned?

As soon as they appear, they roll initiative, so it's somewhat random if they can act the round they're summoned. 

@quindraco is correct any creature that join a combat in progress must roll initiative as normal.