The Ark is a piece of lost technology that Knives uses to absorb all the Plants from Planet Gunsmoke. In its original form, it is a part of a larger fallen ship that Knives has fixed to travel in. After absorbing numerous plants the Ark becomes the center of a larger plant being controlled by Knives. It is eventually taken down by a combined effort of Vash, Earth's Forces, and the collective consciousness of the humans from Gunsmoke.
The Ark fused with the plants
The Ark in its original form