OK, the pain effect is pretty straightforward. Do we want to drop the successful to one round, or negates, perhaps?
Looks pretty simple.
Take our
Nereis conversion.
Increase it to size Large and 2 hit dice but do not change its +1 natural armour, giving it AC11.
EDIT: Scrub that idea, give it +2 natural armour and drop its Dex two points to give it a +0 bonus. This worm should be slow and clumsy, relying on its agonizing venom for defence, not its bristles. :END EDIT
Reduce its bite attack to 1d2 damage.
Remove the burrow speed and change its movement rates to land 10 ft., swim 10 ft.
Remove the special abilities Hold Breath, Improved Grab and Swallow Whole.
Add a pain-causing poison to its Bristles special attack, I propose:
Bristles (Ex): Any creature striking a eurythoe with handheld weapons or natural weapons takes 1d4 points of piercing and slashing damage from the worm's bristles and is injected with an agonizing venom. Note that weapons with exceptional reach, such as longspears, do not endanger their users in this way.
Agonizing venom - DC15 Fortitude save to resist, if the save fails the victim suffers wracking pains that impose a -4 penalty on attack rolls, skill checks and ability checks for 1 minute and then makes a secondary save, failing the secondary save results in the venom's effects lasting for 1d6 [FONT="]×[/FONT]10 minutes. Agonizing venom can be suppressed by the delay poison spell or neutralized by the remove poison spell, or similar effects. The save DC is Constitution-based and includes a +4 racial bonus.