In its true form, a Helghast looks like an undead creature with a rotting face and glowing red eyes, and wears a long hooded cloak. Helghast can, however, disguise themselves as humans and other creatures, and are often used as spies and assassins. Very strong (for a monster encountered at this point in the series), they are also immune to ordinary weapons and can only be damaged by magic weapons. (Because Helghast are undead, the Sommerswerd does them double damage).
Lone Wolf is tracked and attacked by a number of Helghast while attempting to reach Durenor. Most importantly, the tunnel into Hammerdal is overrun with the creatures. There is also a Helghast in the Ragadorn Ale-House Brawl board game.
The word is both singular and plural.