The Alien is a boss mob that has incredible speed.
Aliens drop compasses, iron, maps, and other items when killed, and make an annoying sound on death so if your volume is up really high then you should probably run.
The Alien is quite a tough mob as it runs around ramming into you like a truck, it is recommended to fight it in a tight space as he has no room to roam around making it easier to hit him. It is important to have amethyst armor or higher with a high damage sword to kill him with. Aliens are huge, so a good strategy may be to make a small shelter with a 2 wide door, as the alien would not be able to fit through the entrance. The Alien deals 12 Damage, has 8 Defence and has 100 Health.
The Alien only spawns deep in the Red Ant dimension caves, can be spawned by an ancient dry alien spawn to convert into an alien spawn egg or by using Creative mode.