Rotfiends

Course it reeks. Think they're called rotfiends because they smell like roses?
– Vesemir, witcher of the Wolf School

Rotfiends resemble decomposing human bodies that have been stripped of their skin. Their presence is given away by the overwhelming stench of the rot which gives them their name. Devourers are a particularly dangerous kind of rotfiend marked by an insatiable appetite for human flesh.

Rotfiends and devourers were once rarities, but in the present age of constant warfare and violence they have become a veritable plague, particularly around battlefields and in disease-stricken areas. Though they feed mainly on carrion, they will at times attack the living. They usually feed in large groups and thus present a danger to lone travelers – especially considering their speed, which is more than a match for a horse at full gallop.

The rotfiend's decomposing body is filled with gasses which are poisonous even to those who, like witchers, are immune to most other toxins. These emissions are also highly flammable, meaning any spark, not to mention a carelessly-cast Igni sign, can lead to an explosion. This is particularly likely after a rotfiend dies, when its body thrashes around in uncontrollable tremors.

Rotfiends and devourers feed in the twilight hours and at night, when they become much more dangerous than during the day.