The Pith. Its the building to the right, though only its leg and mouth can be seen.
The Pith is the HQ for the Myriad. It looks similar to a spider with 4 legs and two mouths (one small one on the forehead and a big one below). It is mostly used for training Grubs and researching. Without the Pith, you will get beaten up every single round even if the difficulty is set to "Rookie", so apart from the Battlemite, the Pith is the most important building in a Myriad match.