The Althing was an arena in Asgard.
After defeating Surtur and gaining Scabrite, Thor returned to Asgard, which was under attack from both the Frost Giants and the Fire Demons. Loki informed him that Odin was standing against Mangog in the Althing, but advised Thor to aid Heimdall and Sif in retaking Bifrost Bridge. Odin had stated that Mangog could not be defeated until the forces that fed it hatred were destroyed.
With Asgard's enemies defeated, Thor made for the Althing to confront Mangog. Since Loki had advised him to do that which unleashed the creature, Thor brought him to help him defeat it. They found the weakened Odin barely holding his own against it. He urged them to finish the abomination that they had unleashed. Weakened without the hatred of Asgard's enemies fueling it, Mangog was vulnerable to the power of Scabrite and was destroyed once and for all.