The Sunset Mountains

The Sunset Mountains are inhabited atop by ogres and gnolls and down below by dwarves and drow, with an adult white dragon making its lair amongst the peaks. These small and divided tribes and clans are drastically overshadowed by the power of Darkhold, the Zhentarim capital. Although drastically reduced in power and influence in Faerun since the lost of almost the entirety of their sphere of influence in the Moonsea to the Netherese, the Zhentarim remain a local power in their one remaining stronghold here in the Sunset Mountains. The Zhentarim of Darkhold are mercenaries and slavers, not as vicious as they were in the 14th century but certainly ruthless and willing to side with whoever will grow their power and wealth. As such, seizing the Sunset Mountains means bringing the Zhentarim to heel somehow. The easiest way would be to simply employ them reliably. If the pay is good enough, the Zhent loyalty will not waver, and there are plenty of nearby enemies against which the might of the Zhentish mercenaries would be useful: Elturgard, Najara, Cormyr, the gnolls of the Reaching Wood, the other inhabitants of the Sunset Mountains. Also lurking nearby the Sunset Mountains is the Well of Dragons, the stronghold of the Cult of the Dragon, which seeks to summon Tiamat to destroy the world (they imagine they will rule over what rises from the ashes). Particularly after the Draakhorn is employed to summon armies of chromatic dragons, the Cult of the Dragon represents a major military power in the area. Their power is particularly concerning because they are a small number of very durable monsters. If a single dragon fights a cavalry squadron, and half the squadron is killed and the dragon is halfway killed and both break off, that might be a stalemate immediately, but within a few days the dragon will be healed. The cavalry squadron will still be half-dead, unless they're willing to pay thousands of gold to get their fallen comrades raised, something which no army can keep up for very long. Indeed, once they have the Draakhorn's armies on their side, the Cult is a greater military force than even Baldur's Gate or Silverymoon. While they stand no chance against the united forces of the Lord's Alliance, the Well of Dragons is quite a defensible fortress, and storming it quickly would be much more difficult than laying siege to it for months until the Cult is starved out and forced to fight in the open, where their breath weapons are less effective and the Alliance numerical advantage is more easily brought to bear. The upshot of all this being that the Cult is a significant military threat to the Sword Coast who would be extremely difficult to dislodge from their fortress. Just south of the Sunset Mountains is Iriaebor, a city state that stands between Elturgard and Cormyr. Although independent, Iriaebor maintains a close alliance with Elturel. Their economy is wholly dependent upon trade going from Cormyr to Elturgard, as a city state they have little production of their own and can't survive trading with only one of the two partners, which means anyone who controls either of the two nations would exert significant influence over them. Someone who controls both Elturgard and Cormyr would have Iriaebor at their mercy. Further Reading The Well of Dragons is fully detailed in the Rise of Tiamat, and Darkhold is detailed in the 3e source book Lords of Darkness. This source book is out of date, as the Zhentarim have undergone a serious shift in vision and operating procedure since their near-annihilation at the hands of the Netherese, but much of the details of Darkhold's construction and defenses remains accurate.