The Serpent's Horn is an underground group of fanatical Chaosian terrorists who rabidly hate Amberites; their stated goal is to cut off the horn of the Unicorn and present it to the Serpent.
During the hunt for Demogorgon, they ambushed Moriya, Motoko and Davros with a trapped no-magic Shadow full of albino alligators and demon warriors. However, their collective team of Logrus "masters" were unable to stop Motoko from faking them out and turning the magic back on to allow an escape by teleport, while Moriya and Davros held off the alligators and demons.
Later, Serpent's Horn apparently allied themselves with Lord Alars in the concerted effort to kill Morgan that resulted in the death of Lord Alars, and the loss of the Air Patternblade to House Alars.
There was a splinter faction of the Serpent's Horn that served Daffyl, who is actually a shadow of the true Serpent. However, the madness from losing his eye convinced Daffyl that he was the true Serpent; it certainly appeared that this faction believed as much. Daffyl is the shadow Serpent who guarded Merehallow and Justice, and employed this group of nutjobs to regain Justice (after having stolen Merehallow from Alars) and also punish the group that first took Merehallow. Saionji was first captured, while Michelle and Lady Atsuko was taken after an attack on her estate to get Thoric, who escaped.
Then, during the diplomatic meeting at House Tremere, they attacked, probably in conjunction with disgruntled Tremere members who wanted at least Miki and Madoka to pay for Mutsumi's role in killing one of their lords, Gaav. The attack was largely repulsed, but Davros was taken, and it was at this time that it was discovered that the Bride and her Pillars were also targets of the Serpent's Horn.
There was one more attack by the Horn, on Helgram's house in the Courts during the scheduled meeting, but Loewen anticipated this--he moved the meeting to Helgramways, while rigging his Courts residence into a giant trap. Following this, there was finally the opportunity to strike at the Serpent's Horn. After a daring commando raid to rescue the prisoners and a battle with the SH against Amberite forces, House Helgram, and House Nurgal, Daffyl was given back his eye, curing him of his madness. This collapsed the Serpent's Horn, which had grown into a rather strong organization under him, and those who continued to refuse to see the truth now are a bunch of sucky-ass sucks who suck very much. They will likely not ever be a source of real danger any more.