Mooooore aquatic demons!
Wastriliths are Huge-sized with 15 RHD. Ability scores aren't bad, but hardly impressive: 29 strength is the main selling point while every other stat receives a boost between +2 and +8. Cold immunity is good, fire vulnerability is bad, immunity to water-based attacks is niche. SR 20 makes little sense considering these guys' HD, and zero sense when you consider their even higher CRs. 30 ft. land speed is passable, 80 ft. swim speed situational but useful.
Natural attacks are disappointing: a simple bite/claw/claw routine with relatively low damage dice (2d6 and 2d4). The other offensive abilities worth noting are a simple 3d10 breath weapon of boiling water, Water Mastery (+1 on attack and damage rolls if you and your opponent are both touching water).
The main point of interest here: SLAs! At-will Blasphemy, Control Water, Deeper Darkness, Desecrate, Detect Good/Law, Fear, Greater Dispelling, Greater Teleport, Read Magic, Suggestion, Telekinesis, Tongues, Unhallow, Unholy Blight, and Wall of Ice, in addition to 3/day Symbol, Summon Monster IX (restricted to aquatic/water creatures) and Unholy Aura.
The wastrilith has little to over other than its innate magic, but said magic is a pretty interesting package of utility and the occasional offensive spell. For now I'll assign +1 LA, mostly to compensate for the handful of impressive high-level spells.