In Destiny 2 there are spawning doors that are dark and mysterious and the player cannot walk through them, but enemies can. The enemies spawn behind the door and walk through. This is pretty simple to set up.
Create your doorway.
Place a shadow fx in the doorway (you can use transparent or non transparent depending on the effect you want)
Place an invisible thin primitive in the doorway that blocks the way. In its brain put
WHEN [bump][player] DO [collidable][equals][true]
…WHEN [else] DO [collidable][equals][ false]
Place your spawning cube behind the door. The enemies will walk through the doorway to attack your player, but your player cannot walk through the doorway. Put a special effect on the invisible thin primitive for added effect (bubbles is quite good).