Yhwach

The current leader of the Vandenreich, the military organization of the Quincies. About 1000 years ago Yamamoto Genryuusai failed to kill him, although the circumstances remain unknown.

During Ichigo Kurosaki's guaranteed absense, Yhwach mobilizes the elite of his army, the Stern Ritter, and invades Soul Society. He gave them the order to kill all the death gods.

As the invasion proceeds Yhwach encounters Kenpachi Zaraki and the fight between those two begins. Yhwach easily wins over Kenpachi, commenting how weak he was. Before he manages to finish him off, he is interrupted by enraged Yamamoto.

[Spoiler] After Yamamoto kills fake Yhwach, the true one shows up behind him. Yhwach then summons a giant reishi broadsword and bifurcates Yamamoto. Before turning Yamamoto's lifeless body to ashes, Yhwach mocks him saying how weak he became during the time of the peace.

Yhwach is revealed to be the creator of Quincies and his blood flows in every one of them. He's partially responsible for Kurosaki Masaki's death. [/Spoiler]