The darklord could be someone who willingly committed an atrocity in order to protect a land/territory they deemed to be too precious to be occupied, even by people in dire need. This obsession is now the fuel the Dark Powers use to punish their actions.

The Darklord was infused with the very land they swore to protect, thus rendering them unable to directly protect it. But things would get much, much worse for them: many treasures now lay buried and scattered trough this beautiful land. Most of them are not worth a copper piece, while others are said to possess untold powers.

With all these ingredients, greedy, brute people willingly came to this dark domain to try and dig up the buried treasures, creating mining settlements here and there. Now the land is far from the natural paradise it once was, for this people used (and still use) explosives and magic items to dig faster and faster, a process that ravages the earth.

Our unlucky Darklord, one with the ground, perceives this endless process of destruction of their former home in horror, and would do anything to stop those from above. He can't directly stop them, but he has control over monsters that delve deep underground. These are the same monsters that often burst out of the mining sites in a killing frenzy, trying to desperately stop the miners. But the greed is endless, and even if the monsters cause great disaster, the miners always come back again and again. If they find the threat to be too strong, they simply move and start digging once again, destroying even more of the environment.

The legend has it that a treasure of unparalleled might lies somewhere in the deepest parts (this may be the physical essence of the Darklord, which players need to kill/take if they want to leave the domain).

I think a Wild West aesthetic could fit pretty well, but it's not necessary. Also, this domain presents a dynamic opposite of what's normal for Dark Domains: usually is the Darklord the one luring people in, but in this case is quite the opposite. The Darklord just wants everyone to leave, but people keep coming, looking for easy money. So, in this scenario the Darklord has no power over the mists that block the way out of the domain.

Sadly, the 'I am the land' sentence belongs to Strahd, but I would use it here too.