Blood elf

The Blood Elves are a group of Elves who followed Kael'thas and allied themselves with the Illidari and the Naga.

Most of them are in Outland, but some groups still remain on Azeroth, awaiting the return of their leader, Prince Kael'thas Sunstrider.

Background
Upon the fall of Quel'thalas, Kael'thas gathered all the remaining High Elves and renamed them Blood Elves and set out with a regiment of these to aid the Alliance.

After suffering several slights and almost being executed, Kael'thas separated from the Alliance and joined Lady Vashj and later Illidan Stormrage, under the promise that the latter knew a way to cure his people's addiction to the magic.

After Illidan was defeated at the foot of the Frozen Throne by the Death Knight Arthas Menethil, he set out for Outland with the Blood Elves, promising those who remained that he would return with a cure for their addiction to magic.

Differences with the High Elves
The main visual difference between High Elves and Blood Elves is eye color.

While the eyes of the High Elves still glow a particular blue indicating the use of primarily arcane magic, the eyes of the Blood Elves glow a dark green, indicating the contamination of their bodies by fel magic.

In terms of ideology, the Blood Elves are willing to use any means to survive, including the use of fel magic, this leads them to the main discord with the High Elves, who consider them little more than monsters and traitors for this very reason who repudiate anything that has a connection with demons and the Burning Legion, in addition to being aware of Kael'thas' alliance with the naga and with Illidan Stormrage himself.

Blood Elves often use red as their primary color, as they consider it a symbol in honor of the Quel'dorei blood that was spilled in the plague invasion. High Elves usually avoid these colors, since most races, mainly from the Alliance, repudiate Blood Elves and do not want to be confused with them as such.

Although most are disowned by the Quel'dorei, they accept a few Blood Elves into their society, keeping them under proper surveillance.