Page 4 of the comic.
Lei's sister Astral arrives to see the horrifying sight of her own brother literally drowning Atlas into some sort of black flame. She cries out to her brother to regain his sanity and remember the pledge that he made as one of the Lucario clan but right after she tries that, he completely annihilates the entire clan. Elus looks back and becomes even more scared than he already was at the fact that Atlas' arrogance just unleashed the worst thing that any of them have ever seen. Somebody calls out to Astral but......

Anybody who looks at my art can easily find out what Lei is turning into. But what did Lei sacrifice in order to become that thing?