|Access to this zone is granted after completing the fifth task in the Solteris arc, "Confront Emperor Zaarisz". And in order to use the portals here you must complete the sixth task in the same arc: "Where There's a Will".
To get this zone, take the following path. Katta Castrum > Zhisza, the Shissar Sanctuary (portal in the NW of Katta) > Silyssar, New Chelsith
When in Zhisza, just walk north into the tunnel with a ton of bugs. There is a funny looking wall on your right at the end. Click it and it opens like a door where there are a few pillow pads, candles, and a door at the end. That door is the zone-line for Silyssar.
Silyssar was a destroyed habitat that the Combine had given up for lost. The Shissar managed to locate it and erect a new barrier, pumping out the saltwater and claiming the habitat as a hidden haven. Away from prying eyes, the Shissar were able to hone their technology and magic, achieving feats never thought imaginable. From their seat in Silyssar, the snake-men are poised to destroy the Combine Empire.