A_FadeIn
From ZDoom Wiki
A_FadeIn [(float increase_amount)]
Increases the actor's opacity (alpha) by the specified amount. This can be used to slowly fade in some things in a loop. If increase_amount is not provided, it defaults to 0.1.
Examples
actor Ghostball : CacodemonBall
{
renderstyle Translucent
alpha 0.0
States
{
Spawn:
BAL2 AB 4 bright A_FadeIn(0.2)
loop
}
}
This Cacodemon fireball slowly fades into existance when it is fired.
