Legion of Super-Heroes 4 page 07 by Giffen and Gordon

Published 1990.
written by Keith Giffen and Tom and Mary Bierbaum and Al Gordon.

The return of Mon-El

Meanwhile the Time Trapper has a conversation with the other personality trapped within Mon-El, Eltro Gand, a descendant of Mon-El's brother.
The Trapper recalls how Eltro became merged with Mon-El. Dream Girl had foreseen the death of Mon-El and in order to prevent Mon-El dying, Eltro had drugged him and took his place hoping to sacrifice himself for Mon-El. However, Mon-El died because he was unable to take his anti-lead poisoning serum. Eltro then projected his life essence into Mon-El, sacrificing himself and reviving Mon-El. (Adventure Comics #384, 1970).
However, the Time Trapper explained that Mon-El was not dead and what Eltro had done was merge his essence with Mon-El, causing Mon-El fits of depression.