Whereas most Verminlords are explicitly compared to Greater Daemons, Skreech Verminking shares more in common with a Daemon Prince in that his essence was originally mortal. When the Skaven first warred against Nagash, the Council of Thirteen eventually managed to slay the ambitious founder of necromancy. Though initially Impressed by this, the Horned Rat got pissed when the Skaven went right back to their petty rivalries instead of claiming dominion of the world. In rage, he plucked those Lords of Decay from the mortal world and threw them into a dark pit in his realm, where they languished for centuries. In the pit, they could do nothing but blame each other and feed on the scraps of failed plot thrown at them by the Horned Rat. Eventually, the horned one realized that they had still accomplished much, so he dug them out of the pit. By this point they were a horrible mewling mass of 12 rats fused into one, so he blasted the twelve Lords of Decay with a single searing bolt of black warp-lightning, sweeping up their souls and merging them into a singular entity. Thus was born Skreech Verminking, greatest of all verminlords, the purest representation of all the spiritual aspects of Skavendom and truest avatar of the Horned Rat. Not only does this make him a disturbing parallel to a certain gestalt leader, he's also a subtle reference to the Rat King, both the Nutcracker villain and the disgusting real-life occurrence (the latter is gross; essentially a bunch of rats have their tails stuck together by tree sap, hair, or some other kind of sticky or entangling substance).
And of course, only the greatest of Skaven could summon the greatest of Verminlords: Thanquol. Together, the two lay waste to Nuln and eventually Skreech tells the psychotic Grey Seer to make parley with Archaon, leading to the creation of the Grand Legion of the Everchosen. From there...they just got forgotten. According to Josh Reynolds, he and Thanquol manage to relocate Skavenblight into the Warp...which half-fails. But at least they survive to see the coming of their god's rising. Most importantly of all, he has a whole bunch of horns.
Some have theorized he is the Verminlord known as “The Screaming One” from Blood In The Badlands, but since TSO was already in the world whereas it took Thanquol’s immense power in End Times to summon Skreech, its unlikely.
On The Tabletop
On the tabletop, Skreech Verminking has the exact same attributes as any Verminlord, but costs a good 150 points more. His "affinity spell" (the automatic spell he starts with and which he can re-roll failed casting attempts for) is The Dreaded Thirteenth Spell, and he wields the Doom Glaive (Multiple Wounds D3) and Plaguereaper (Always Strikes First) magic weapons, though he only benefits from one in each Close Combat Phase.