Taeshalach is the sword of the titan Aggramar. When Sargeras betrayed the Pantheon, Aggramar met him in a duel. When their blades met, it ignited an explosion of fel and arcane that caused both weapons to shatter. As such, only half of Taeshalach remains physically intact, with the other half being made up of flame. In Aggramar's Antorus encounter, Taeshalach could summon fire elementals called the Embers, Flames, and Manifestations of Taeshalach.
In the RPG
Taeshalach (meaning "Flame Rend") is a sword that was forged from one of the two Shatterbound fragments of Gorshalach, and embodies all that is noble and good. When Gorshalach sundered itself, Sargeras abandoned the fragment that would form Taeshalach, as it was useless to him in his corrupted state. Aggramar eventually claimed the fragment and reforged it into its current form, which he currently wields as his weapon. If this weapon is ever combined with the other fragment, which has been reforged into Gorribal, Gorshalach will be reborn.
- Taeshalach's origins in canon are completely different from those of the non-canonical RPG, as in canon, Sargeras and Aggramar's weapons were always different, separate blades.
Taeshalach in Shadows & Light.