Sectoids are the face of aliens, not just in XCOM 2, but also other medium that presents alien. It has the classic look of alien but this time, has a humanoid posture and body. Normally, Sectoids do not pose a threat with its weaponry. However, their Psi abilities punish you for ignoring them for reasons even this world will not understand.
Mindspin: This is a roulette of psi-effects that can cause only one of the following: panic, disorientation, or mind-control.
Reanimate: Reanimate ANY organic soldier, friend, foe or civilian! This will turn that fallen unit into a zombie retaining the health pool of its living counterpart, but is removed with all of its capabilities and attacks.
Panic: Induce Panic for two turns.
Mind Control: Well, mind control!
Sectoids are very vulnerable to grenades, all of types of grenades.