Thanos was initially playable back in 2019, to celebrate the release of Avengers Endgame, where you could play as him as part of the Infinity Gauntlet Limited Time event.
Once you found the Infinity Gauntlet, you'd transform in Thanos and were left to cause chaos across the island. He's was pretty OP at the time.
Now, via a cryptic but pretty easy to decipher tweet, Epic Games has teased his return.
Update - Thanos Cup Confirmed
Following the teases, the Thanos Cup has been officially revealed and you can check out all of the details here.
"Three years after his LTM debut in Fortnite, Thanos arrives in the Item Shop for the very first time. Beginning at 8pm ET on June 26, 2021, you can play as the Mad Titan himself, even equipping the Infinity Gauntlet Back Bling inspired by Marvel Studios’ Avengers: Endgame - completed with all six Infinity Stones."
"Before Thanos arrives in the Item Shop, players will have the opportunity to get his Outfit and Back Bling inspired by Marvel Studios’ Avengers: Endgame first by competing in the Thanos Cup on June 21!"
Thanos Returns to Fortnite
As you can see from the tweet from Epic's official account below, they've used Thanos' "I'm inevitable" quote and the colours of the five Infinity Stones.
However, it's not clear how exactly he'll return - whether it will be as a cosmetic or as a playable character once more.
Thanos Cup Tournament
However, as iFireMonkey pointed out on Twitter, a new URL has gone up on Epic Games' website that hints at a tournament called the Thanos Cup.
When you load up the link, it will briefly says "Thanos is coming, get him first in the Thanos Cup!" before redirecting to a 404 link that's yet to go live.
When that tournament will start, and how it'll work, we have no idea yet, but the URL being live suggests that the reveal is imminent.
As the URL says "get him first," it seems like Thanos will be there as a reward more so than a playable character.
Skins often get given out as early rewards for doing well in themed cups, so the case could be the same here. Obviously that's still unconfirmed, though.
We'll let you know as soon as we have some more concrete information on it.