Acala (Achala) was one of the attending warriors who aided Skanda-Kārtikeya (Skanda-Kartikeya) at the time of killing Tārakāsura (Tarakasura). 

Mahābhārata (Kinjawadekar)(9.45.74)
Mahābhārata Vols. 1-43 (Siddhāntabāgīśa)(See footnotes of verse number 9.42.52; Vol 29; Page number 476)