Thor's enchanted hammer Mjolnir is returning in Thor: Love & Thunder, but with a very different appearance. For centuries, Thor believed Mjolnir was the key to his might. It was only in Thor: Ragnarok that he learned this was not the case; rather, the hammer served as a channel for his power. He only learned this truth after Mjolnir was destroyed by Hela, his elder sister, leaving him shaken at the loss.
But the Thor: Love & Thunder trailer confirms Mjolnir is returning - in the hands of Thor's successor. Possession of the hammer grants anyone who is worthy the power of Thor himself, and now it has been claimed by Thor's ex, Jane Foster (played by Natalie Portman). She's shown holding it aloft, with Thor Odinson watching in amazement; although this may not be Foster's first scene as the Mighty Thor, it seems to have been her first encounter with the Odinson. She's masked, which means that - as in the comics - Thor may well have no idea who his successor really is.
The once-shattered Mjolnir has changed, however. No longer is the hammer a perfect one; now, its surface is marked by deep cracks showing where the pieces have been put back together, and blue arcs of lightning can be seen shining out of them. The general assumption is that Jane Foster will reforge the Mjolnir broken in Ragnarok, but the visual design of the hammer seems to suggest that may not be the case; rather, it's possible the blue energy itself is what is holding the pieces in place, explaining why the cracks are still visible.
The interesting question, of course, is how Mjolnir is reforged in Thor: Love & Thunder. In the comics, Mjolnir has been destroyed many times - most notably during Jason Aaron's Thor run, which is serving as a major source of inspiration for this arc. On every occasion, it's been reforged, sometimes at great cost; once Thor even used the sun itself as a forge in which to remake his hammer. Jane Foster is unlikely to possess the ability to reforge Mjolnir, however, and there's no hint Eitri - last of the Dwarves, the only craftsman likely to be able to do so - is in Thor: Love & Thunder. That raises the possibility Mjolnir's enchantment still holds.
In the first Thor film, Odin placed a worthiness enchantment upon Mjolnir. From that moment on, whenever the hammer was in the presence of someone it deemed worthy, it would grant them the power of Thor - as seen in Avengers: Endgame, when Captain America used a version of Mjolnir retrieved from the past. Most viewers had assumed that enchantment was destroyed when Hela crushed Mjolnir, but it's possible it has endured. If that is the case, the presence of someone who is worthy could have been enough to activate the magic, causing Mjolnir to reform and grant Jane Foster the power of Thor. That would explain why the energy effect in the Thor: Love & Thunder trailer seems to be seeping out of the cracks, and it would be a smart inversion of Thor Odinson's arc in Thor: Ragnarok. In Jane Foster's case, the hammer really is the source of her power.
