Avernus, the first Layer of Baator, is blood, violence, domination, and death. Most of it is a rocky wasteland. Beneath the blood-red sky balls of fire burst unpredictably. They are said to be embodiments of the rage of the imprisoned Lord of the First, Zariel.
The River Styx flows through this layer, as does the appropriately named River of Blood.
From the gate in Ribcage the nearest site is the ruined city of Darkspine, where refugees desperately try to avoid the baatezu thought police while trying to do enough good to shift their town back to the Outlands.
In the center of it all is the Bronze Citadel of Bel, Warlord of Avernus, a tangled and continually growing collection of walls and buildings extending from the ancient fortress of the imprisoned ruler of the plane.
The largest series of astral conduits on Baator lead to the Maggot Pit, a sea of squirming infernal larvae, beyond which the Dragonspawn Pits of Azharul sprawl in a cluster of tall hills and mountains. Beside the Pillar of Skulls is the gateway to Dis, guarded by Tiamat and her draconic and Abishai servitors.
This Layer is also the location of Abthalom, the "Abyss" of Takhisis (not to be confused with the Abyss). Vast legions of Abishai serve the Queen of Darkness, milling through the Rallying Grounds and near the Temple of Takhisis and the Tower of High Sorcery.
Also in Avernus is Draukari, the realm of the kobold god Kurtulmak, who is said to be a brother or son of Tiamat. The towns in Draukari, hidden in the hills, mountains, and deep forests of the realm, are Frekstavik, Nibellin, and Snjarll. Kurtulmak's realm is often raided by goblins serving Bargrivyek, whose own realm "the Peaceable Lands" is nearby.
Garden seems to be an oasis, with fountains of delicious water and trees that put Arborea's to shame. Many are the explorers who brave the terrors of Avernus in the chance of finding it, but many of those who do are never heard from again. Is the Garden a refuge for souls that reached Baator by mistake, a lost paradise hidden by the gods, or a sinister lie?