A_FadeOut
From ZDoom Wiki
A_FadeOut [(float reduce_amount[, bool remove])]
Increases the actor's translucency (alpha) by the specified amount. When the actor is fully transparent it will be removed, unless remove is explicitly set to false (it is true by default). This can be used to slowly fade out some things in a loop. If reduce_amount is not provided, it defaults to 0.1.
Example
actor Fadeball : CacodemonBall { renderstyle Translucent alpha 1.0 States { Spawn: BAL2 AB 4 bright A_FadeOut(0.2) loop } }
This is a Cacodemon fireball which slowly fades out of existence when it is fired.