Glathar was a sword that belonged to King Vortigern. After Vortigern's death the sword was kept in the vaults of Camelot. Morgause le Fay stole it and enchanted it before the Battle of Camlann, then gave it to Mordred for him to use. It injured Merlin and mortally wounded Arthur Pendragon. Dubbed "The Coward's Blade" by Merlin because he thought of Mordred as a coward for killing a just man.
Glathar is a roman-style sword. It lacks a crossguard. It is about three feet long. Morgause enchanted the sword to fatally wound any living thing it touches, though it was still not as powerful as a sword forged in a dragon's breath and the enchantment had no effect on Merlin.