Tremorsense allows a monster to feel the vibrations through a solid surface caused by movement. It is an imprecise sense with a limited range (listed in the ability). Tremorsense functions only if the monster is on the same surface as the subject, and only if the subject is moving along (or burrowing through) the surface.
AC 31; Fort +21, Ref +18, Will +24; +2 status to all saves vs. magic
HP 140
+2 Status to All Saves vs. Magic
Inexorable
When Narseigus gains the Paralyzed, Slowed, or Stunned condition, roll a DC 15 flat check. On a success, he ignores the condition.
Light Blindness
When first exposed to bright light, the monster is Blinded until the end of its next turn. After this exposure, light doesn't blind the monster again until after it spends 1 hour in darkness. However, as long as the monster is in an area of bright light, it's Dazzled.
Shake It Off(fortune, primal)
Frequency once per day
Trigger Narseigus fails or critically fails a saving throw against a condition or adverse effect (such as Cursed Metamorphosis)
Effect Narseigus rerolls the triggering saving throw and uses the better result.
Speed 20 feet, burrow 20 feet, climb 20 feet
Melee Staff +20 (magical, two hand d8), Damage 2d4+8 bludgeoning
If Narseigus' next action is to cast an occult spontaneous cantrip or an occult spontaneous spell of 4th level or lower, he reduces the number of actions to cast it by 1 (minimum 1 action).
Steady Spellcasting
If a reaction would disrupt Narseigus' spellcasting action, he attempts a DC 15 flat check. On a success, the action isn't disrupted.
Worm Form(morph, occult)
Narseigus partially transforms his lower body into that of a cave worm. He can use only his tail unarmed Strike and his burrow and climb Speeds while in this form. This transformation lasts for 1 minute.
Worm's Feast(primal)
Frequency once per day
Effect Narseigus calls forth a brood of infantile cave worms to feast on his enemy's flesh. One creature within 30 feet must attempt a DC 27 Fortitude save.
Critical Success The worms writhe away harmlessly.
Success The target takes 2d6 piercing damage and 4d6 persistent piercing damage.
Failure The target takes 6d6 piercing damage and 4d6 persistent piercing damage.
Critical Failure As failure, plus the target is Off-Guard as long as it takes persistent piercing damage.
Note: A DC for this ability was not provided by Paizo. The DC present here is a moderate DC for the creature's level based on the Gamemastery Guide creature building rules