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 the following:

alien_warpin_effect.jmf & alien_warpin_effect.map for non-Jack users

Both of these files contain three alien warp-in prefabs, one for each size of monster:
small, for small monsters such as headcrabs
medium, for medium sized monsters such as vortigaunts, houndeye, bullsquid
large, for large monsters such as the alien grunt

alien_warpin_effect.bsp

A playable bsp demonstrating the three different warpball sizes for the three different monster sizes.

alien_warpin_prefabs.jmf & alien_warpin_prefabs.map for non-Jack users

These files contain 16 preset alien warp-in prefabs for each of the three different monster sizes. All you need to do if you wish to have a monster warp in in your map, is open the jmf/.map in your editor, select the prefab of the desired monster size (note that all entities have been grouped) and then paste it into your working map. You may need to open up the monstermaker properties if you want to change which monster to spawn. By default, the small warp-in prefabs are set to spawn headcrabs, the medium sized warp-in prefabs are set to spawn vortigaunt and the large warp-in prefabs are set to spawn alien grunts.
Note that I've added a floor to the map, and you'll see that all monstermaker entities (except for the headcrab monstermakers) are placed exactly on top of the floor, that is, the monstermaker center origin point sits on top of the floor. This stops the spawned monster from falling, and instead will spawn on the floor without falling or getting stuck. If you do want the monster to fall down after it as spawned in, then of course place the prefab higher above the floor. Make sure you don't only move the monstermaker up, but every entity that makes up the prefab. Keep each individual warp-in prefab grouped and enable Ignore Groups (ig button in the top toolbar) if you wish to change entity properties.

To trigger the warp-in effect, target the multimanager.

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Version history:

December 15, 2022
- Removed Hammer library file
- Now includes three different warpball sizes for three different monster sizes
- Now comes with 16 preset alien warp-in prefabs in three different sizes, for quick and easy copy and paste
- Removed useless dummy monstermaker entity

January 4, 2013
- Fixed dumb typo

March 11, 2012
- New and improved alien warp-in effect released