The Hogwarts Hellons are a group led by Liza Fey. They could be likened to Death Eaters, and Liza to Voldermort, except the aims are the two groups are different.
The Hogwarts Hellions, shortened simply to 'The Hellions', want to eradicate Blood Prejudice, and want to keep the peace...by any means needed. They are not above using violence, blackmail and extortion to get what they want, though most of the more extreme action is only taken as ordered by the Queen Snake, Liza Fey.
For the most part, the Hellions are just Hogwarts students who intervene if they spot a 'situation', and report to the Queen Snake. They follow orders without question.
All Hellions have a magical tattoo, the 'Reaper Call'. This is a normally black tattoo that turns crimson and burns the skin when Liza uses the incantation 'Messor Dico' on her tattoo. This is a signal for all Hellions to apply their disguises and apparate to Liza's side.
The disguise consists of a Red Robe with the Reaper Call logo on one side and 'Hell Hath No Fury...' written on the other, and a demon mask, though only Liza's horns show over her robe hood, so she can be identified as the leader. It is applied with the incantation 'Abyssus Dissimulo'