The Stygian Veil, commonly known as "veil", or abbreviated to just "v", is one of the four sections of DoA. Most of the enemies here drop Stygian Gemstones, although some drop Torments. The environment effect is Demonic Miasma.
There are 5 hills to the south and east of the Dreadspawn Maw.
The south-west one, known as "ranger hill", contains a lot of Stygian mobs, including the ranger Underlord, and 2 Water Tormentors. The north-east one, known as "derv hill", also contains a lot of Stygian mobs, including the dervish Underlord, and many pop-up groups of Stygians and Torment Claws.
The three middle hills do not initially contain any enemies, but groups spawn on each hill during 360.
The Trench is a series of narrow passages leading all around Veil. It consists of the back trench, where all four Stygian Lords are found, and 5 smaller trenches radiating from the Dreadspawn Maw, one leading to each Lord, and one towards Gloom.
The 5 are known by the destination at the far end from the Maw, e.g. "monk trench" and "Gloom trench". Walking some distance down any of them will "pop the trench", causing a group of melee Stygians to spawn at each end of the trench and run to the far end, while 2 random Tormentors (always a Water, Soul, or Mind) spawn on the hills overlooking the trench.
The back trench stretches from the end of the Gloom trench all the way around to the Dreadspawn Maw, although the gate at the Maw is permanently closed. There are 11 groups, each consisting of Stygians and Tormentors, patrolling back and forth throughout the trench.