| Orthos |
Problem is, IHN, that we have stats for these creatures and can compare them.
Both are CR 20 threats.
The Jabberwock has fire-damage eye rays, which negate the Starspawn's regeneration.
The Starspawn does NOT have a Vorpal weapon to get through the Jabberwock's DR.
The Jabberwock will win through attrition alone. The Starspawn's damage will be significantly reduced (DR 15/vorpal AND Fast Healing 15 vs. 2d6+16 on Starspawn's best attack), so it'll be doing quite a lot less harm. And the Starspawn has nothing to prevent the dragon from hitting back with the full force of its attacks. Heck, the Starspawn isn't even immune to mind-affecting affects! The Jabberwock can still Burble at it and possibly confuse it! (With Mind Blank a constant, the chance is really, really low, but it's there. I'm really very surprised to see the Spawn not have that immunity outright.)
Its only hope is that it can get lucky with some of its spell-likes and incapacitate the Jabberwock early on. (A difficult task, given the Jabberwock's saves - Will +22 versus DCs 24, 22, and 25 on the Starspawn's SLAs. It *could* get lucky, the Jab *could* roll a 1, but it's unlikely.) If it can't manage that, it's done for.