How can I make this effect?

[Seitan]

The white flash when you get teleported into Hightower_event's Underworld.

Just like this:

View: https://youtu.be/ignrzK-xQts?t=48

 

[Seitan]

I already decompiled helltower but I can't find any entity who can do a effect like that

 

[nickybakes]

That's an env_fade. You can use it to cover the screen to any color you want, and there's some settings you can use to customize how it shows up in game. You can click the "help" button on the entity's properties window, or view the VDC, for help on these properties.

https://developer.valvesoftware.com/wiki/Env_fade