The Thor's Stormbreaker is a special Mythic weapon that can only be found in the Avengers: Endgame LTM. You can swing the axe to deal damage to opponents at close range using the shooting button, You can also use the shoot button while in air to leap to targets/opponents and this behavior is similar to Thanos, You can even throw the axe to destroy structures from far away or eliminate opponents from long distance.
Despite being a storm/lightning axe from the movie, it does not summon any storm or lightning in the Avengers: Endgame LTM.
This item works similarly to the Infinity Blade where you can leap and deal damage to opponents and also swing the sword to use it at close range, however you cannot throw the Infinity Blade.
When you eliminate someone with Thor's Stormbreaker's Throw attack, the kill feed says "[Player] went for the head of [Player] with Stormbreaker!", which is a reference to the Marvel film Avengers Infinity War, where during the ending, Thor, one of the film's protagonists, struck a blow to the body of the main villain Thanos which failed to kill him, giving him time to regenerate and leading him to say that line.
This item along with the other Marvel items is tied to the Avengers: Endgame Movie (2019), as a collaboration between Marvel and Epic Games.