Yes, let's just make the "temporary HD" into 24-hr negative levels. And capping at 10HD is fine.
Actually, yes, I was thinking of 10 negative levels, I guess. But any of those works. The last is probably best.
That would be just fine with me.
Want to change the whisper or wail, or are we happy with them?
Whisper (Sp): Soul beckoners possess a sonic charm attack that reaches a range of 240 feet. Creatures hearing the whispers must roll a successful Will save (DC 14) or be drawn toward the soul beckoner. A successful saving throw negates the whisper. Creatures drawn to a soul beckoner are attacked by the monster and are considered flat-footed (even if they are normally immune to being caught flat-footed), but a successful hit breaks the enchantment of the whisper. Otherwise, victims are allowed a Will save every round to escape the creature's enchantment with a cumulative +2 bonus per round. A silence spell or a character incapable of hearing prevents the effects of the whisper.
Eerie Wail (Sp): If a soul beckoner’s whisper attack fails, the victim hears an eerie wailing sound. The character must then roll another successful Will save (DC 14) or flee in terror for 1d4+1 rounds. A silence spell or a character incapable of hearing prevents the effects of the wail.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.