"Unlock gate" is an objective in the quest Unlock Gate in T.K. Baha's Bloody Harvest DLC
Head on through The Infested Forge through to the The Lavabluffs. You will find Spiderants waiting outside the Forge and in the Lavabluffs you have more Reanimated Bandits coming after you.
As you head through the Lavabluffs, you will need to make a jump across a collapsed section of bridge. Just beyond the bridge, to the right, you will find a massive cauldron. This is the Sacrificial Cauldron. Keep in mind if you have things you want to get rid of. Otherwise, just head on through Lavashore Village back to the Necropolis and the gate with the Mystic Lock.