This is the alien warp-in effect we've seen so much in Half-Life, it's official expansion packs and of course many mods as well.
In the zip file you'll find a rmf file that you can compile to see the effect in-game. You will also find a prefab library file (.ol) that you can place in your prefabs directory within your Hammer directory. This library contains 20 alien warp-in presets that you can instantly place in your maps. This will save you the tedious work of renaming all the entities if you want to have more than one warp-in effect in your map. They all spawn an alien slave/vortigaunt by default so that's something you'll need to change if you want to spawn a different monster. Other than that, all you need to do is trigger one entity to get the effect going. Just look at the rmf file to see which one you'll need to trigger.
11th March, 2012 - New and improved alien warp-in effect released 4th January, 2013 - Fixed dumb typo.