The Blood of Ymir was an ancient Asgardian relic stored in a hidden chamber frozen in ice within Odin's palace. When Thor sought Odin's help to fend off an invasion from Dark Celestials, Odin gave him access to the chamber of the Blood of Ymir, claiming it was a powerful weapon that could help fight the Dark Celestials. Thor and his fellow Avenger She-Hulk made their way through a deadly blizzard and managed to retrieve it from inside.
After retrieving the artifact, Thor and She-Hulk returned to Earth and joined their fellow Avengers in the final battle against the Dark Celestials. The pair consumed the Blood of Ymir and grew to the size of Celestials. Thor and She-Hulk battled the Dark Celestials using their newfound power, but it was only temporary and they shrank back down to their original sizes.
The Blood of Ymir, upon being consumed, causes its consumer to gain the size and power of a Celestial. The only downside is that the power provided is temporary.