When Acelin is paired with at least one other faction ally, his passive states that allies that are under Shield buff are immune to Fear and True Fear debuffs.

If the Head of Torment applies the Fear or True Fear debuff this is applied correctly as the Torment Head's passive states that this cannot be blocked or resisted. 

However, the issue is that the effect of the Fear or True Fear can still activate and cause the loss of turn (and cooldown of skill if it is True Fear), based on the wording of Immune in Acelin's passive the Fear/True Fear should not activate even if a champion has this debuff and is also under a Shield buff.