Viskardis

Viskardis, the Queen Retriever, Jailer of the Undercourts, and Prince of the 796th Layer

The Demon Prince of all Retrievers is a titanic, colossal construct with individual legs each hundreds of feet long and dozens of feet in diameter. Her body, as long and wide as an entire city, is made of a metal unknown in the realms of mortals, hardened like adamantine and sharpened to wicked points the size of church steeples. And her sable eyes - the size of sailing ships - are rumored to freeze or flay dozens of hapless creatures at once in a cone of relentless magic.
Viskardis was forged by the Abyss's greatest crafters as a sentient prison that would walk about the lines of the Blood War and permanently detain Devils who were too important to kill. Her presence over a battlefield, progressing inexorably overhead like an eight legged thunderstorm, often gave pause to even the most hardened hellish warriors. But she was an easy target for raids, constantly under attack by Devils who would teleport upon her back to find or free their commanders. Viskardis herself was never felled by these raids, but the other Demon Princes realized that her value was too great for the battlefield.
So instead, they crafted for her an infinite realm inhospitable to even outsiders, The Black Web. This abyssal layer had no atmosphere of its own, and was enchanted to constantly shed the deadliest poison the abyss could conjure. They granted her sole dominion, and the resources to craft her own legions of loyal retrievers, who she often leases to other Demons for boons. But no service she provides is greater than her prison, which is now so far out of sight of Hell that any attempts to raid it have become the venture of fools.