Immovable Warrior only specifies that it can't be moved with regards to the wearer. The wearer has no such defense.

Anyway...

Omphalos Wyrm

You see before you a worm the length of a man's spine, jet black with yellow splotches. It looks dead; however, when you move to pick it up, it swings around quick as a whip and burrows into your back. You feel... stronger, somehow.

The Omphalos Wyrm is a living Minor Artifact, with an Intelligence, Charisma, and Wisdom score of 20 each. Once it has burrowed into a creature's body, it replaces their spine, and starts putting out tendrils that infiltrate the nervous system of the host. This grants their host a +2 Perfection bonus to Con and Wis, as well as improving their natural armor bonus by 2 and doubling their rate of natural healing. However, this is not the only boon the host gets.

At will, as a Swift action, the host may allow the Omphalos Wyrm to expand, consuming muscle and skin, until they look like a hulking jet black salamander-man with yellow spots. While in this form, you gain a +16 Perfection bonus to Strength, as well as a +10 Perfection bonus to Dexterity and Constitution; additionally, your flesh drags itself together, granting you Fast Healing 5, a +10 bonus to your Natural Armor, and DR 10/-.

Leaving the Omphalos form requires a minute as the Omphalos Wyrm reconstitutes your body from its memories; upon exiting the Omphalos form, you are targeted by a Regeneration effect.

However, not all is fun and games; the Omphalos Wyrm has its own rather inscrutable goals. Each time you awake, you must make a DC 25 Will save; if you fail this save, the Omphalos Wyrm takes control of your body, immediately forcing you into Omphalos form.