Must be activated from the Graveyard. Target 1 'Aesir' monster you control; this turn, it is unaffected by the effects of other Spells/Traps. During your 3rd End Phase after activation: Inflict damage to your opponent equal to the combined ATK of all monsters you control, and if you do, banish this card and all monsters you control.
Gjallarhorn essentially gives any Aesir monster Odin, Father of the Aesir’s protection effect. And you are almost guaranteed to win if the Aesir monster is present when Gjallahorn’s burn effect is activated, which is made easier with their self revival effects.
Can't make my mind up on this card. Yes it will nearly certaintly OTK when it eventually triggers, but it's not going to be quickly done and if you are relying on it then you are drawing out the duel longer.