Player death at the end of level E1M8, Phobos Anomaly:

After the player kills the two Barons of Hell at the end of
E1M8, the walls of the star-shaped room drop, allowing
access to an outside teleporter.
This teleporter transports the player to a small, dark room
with a green-acid floor, and the player is surrounded by
monsters.  Here, the player character appears to be killed
before the end of the Episode.

For better or worse, this is the NORMAL ending for this
Episode of Doom 1 and Ultimate Doom.  This is true both for
the shareware version, and the full, commercial version.

Short of quitting the game, or using cheat codes, there is
no other way to exit the level successfully.