John Dee.

"Dee destroyed Yggdrasill," Odin said, reverting to a language that predated the arrival of the humani by millenia. "He killed Hekate."

There was the sudden stink of burnt meat and a purple-red shimmer darkened the Elder's flesh. "Ah, my dear wife forgot to tell me that. A humani killed Hekate?" Mars Ultor asked, voice trembling with rage. "Your Hekate?" he said to Odin.

The Elder nodded. "My Hekate," he whispered.

"And destroyed the Yggdrasill," Hel repeated. "The Shadowrealms of Asgard, Niflheim and the World of Darkness were destroyed. The gates to another six worlds have collapsed, sealing them off forever, dooming them to stagnation and destruction." 

"One man did this?" Mars asked.

"The humani Dee," Hel said.