A_FaceTarget
From ZDoom Wiki
A_FaceTarget [(float angle[, float pitch])]
Changes the calling actor's angle to face its current target.
If angle is specified, then the calling actor cannot turn by more than said angle. (A value of 0 is interpreted as unlimited angle.) However, the SHADOW flag has no effect in such case.
If pitch is specified to a value no greater than 180, then the calling actor's pitch is adjusted by up to said value to face its current target. (A value of 0 is interpreted as unlimited angle; and technically a pitch change will never be greater than 180 degrees.)
Examples
Almost every monster uses this function before it shoots at its target.
Missile: TROO EF 8 A_FaceTarget TROO G 6 A_TroopAttack // See DoomImpBall goto See
This lost soul will home in on its target by calling A_FaceTarget repeatedly.
actor HomingLostSoul : LostSoul { States { Missile: SKUL C 10 bright A_FaceTarget SKUL D 4 bright A_SkullAttack SKUL CD 4 bright SKUL C 0 bright A_FaceTarget goto Missile+2 } }