If you're asking what mother dipped her son in the River Styx, the answer is: Thetis. If you're asking who got dipped in the River Styx by his mother, the answer is: Achilles.
Achilles was dipped into the River Styx.
achillies
She dipped him in the river styx.
For one, He wasn't "Dipped in immortal" he was dipped in the river called Styx. Also he was being held by his ankle, therefore his ankle wasn't dipped in the River.
Thetis dipped her son, Achilles into the Styx river, in hopes of making him invulnerable. However, she held onto him by his heel, therefore making him vulnerable only on his heel.
Thetis (Achilles' mother) dipped Achilles into the River Styx, located in the Underworld
When he was a baby his mother dipped him into the River Styx. Yet she held him by his heel which therefore did not gain the powers of invincibility provided by the waters of the River Styx.
His mother dipped him in the River Styx, but because she held on to him by the heel, that is the spot where he was vulnerable.
His vulnerable heel, which his mother did not dip in the River Styx when she sought to give him invulnerability when she dipped him in the river as a child.
Thetis was Achilles mother. She dipped Achilles into the River Styx, by holding him by his ankles. As the water failed to touch where his mother held him, his heels proved to be his weak spot.
His heel. The legend says he was dipped in the river Styx by his foot and made him invincible. Except his foot was never dipped in so when he was hit by an arrow in the foot he was killed.