The hp for the bosses in Harrowhold is displayed via the "Demoros (default?) gauge." This means that the buffs and debuffs placed on the bosses are not visible through normal means. For most of the phases, this is only an annoyance, that can be played around with enough knowledge of the dungeon (it makes it very difficult for mystics to coordinate contagion on a single target, for example).
However, on phase 4, there is a mechanic where each healer must plague/regress the boss (5 times total) at very specific times, which are only indicated by watching stacks of a shield-looking buff on the bosses bar (plague/regress at 3 stacks). Since the bosses buffs and debuffs are not visible, there is no way to normally know when to plague/regress. Failing this mechanic is not an option if you want to clear, because it makes the boss faster (more difficult to survive) and tankier (he's too tanky on phase 4 to begin with).
Please, can we have the boss hp bars be visible in Harrowhold? Or at least some kind of fix that tells you when to plague/regress on phase 4...