Mallus

Mallus was a Demon overlord that battled the Legends, helped by Damien Darkh.

The Flash: Season 5
Wally West recounted the events that led up to Mallus' defeat to his father when the latter asked what was bothering Wally. After realizing what Wally had to go through against Mallus, it indirectly led to Joe realizing who Wally missed the most.