All solos use (level x 8 + 8 + Con) x 4 or 5 to determine hit points, rather than the role dependent hit points for standard monsters. So the bebilith, heroslayer hydra, and adamantine dragon all have x4 hit points, as per heroic tier solos.

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