A_FadeOut
From ZDoom Wiki
A_FadeOut [(float reduce_amount)]
Increases the actor's translucency (alpha) by the specified amount. When the actor is fully transparent it will be removed. 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 existance when it is fired.
