The Phazon Suit is the result of Samus Aran's Power Suit being infused with Phazon. It has extremely high armor strength and it is immune to Phazon radiation. Samus wore this suit later in her mission on Tallon IV, up to the final battle against Metroid Prime.
Samus acquired the Phazon Suit after killing the Omega Pirate deep in the Phazon Mines. The huge Pirate supersoldier fell on top of Samus, the Phazon in its body dissolving it. As it melted away, Samus was immersed in liquid Phazon which then began to bind with the Power Suit. When it was over, the suit had been greatly enhanced by the Phazon infusion. She would then wear the Phazon Suit for the remainder of the mission.
The Phazon Suit's armor is deep black in color, and a shimmering aura can be seen around Samus, which could likely be a Phazon energy field that shields her from Phazon radiation. The lights on her suit glow red, and they now softly blink on and off, unlike her previous suits which had lights that kept glowing indefinitely. The visor's tint is now a harsh orange.
As mentioned, the Phazon Suit does not take damage from Phazon radiation, allowing Samus to safely traverse Phazon-covered terrain, with the notable exception of the "Orange Phazon" in the Impact Crater. Also, the suit can channel Phazon energy when standing in pure Phazon and use that energy to fire a Hyper Beam-like attack called the Phazon Beam.
When Samus defeated Metroid Prime's core esscence, the Phazon entity began to absorb all Phazon in the area in a last bid to save itself. Sensing the Phazon within the Power Suit, Metroid Prime grabbed Samus and tore the Phazon right off her. After it violently exploded from Phazon overload, a pool of Phazon was left in Metroid Prime's lair. A strange creature clad in a black power suit came forth from the pool, which was revealed to be Dark Samus, the reincarnation of Metroid Prime that took the form of Samus as a side effect of absorbing the Phazon Suit.