Tal'ashar is an ancient rite within the Nightborne civilization, the invocation of which leads to an honor duel between the accused and accuser (or representatives of one or the other). The duel is to the death.
Tal'ashar is known to be practiced among the aristocracy of Suramar City, generally in response to a perceived slight or for competitive reasons. The challenge may be issued over a particular prize, such as a title, with the winner claiming all. Either participant in the rite may choose to present a champion to fight in their place, representing them during the duel. Presumably, all forms of weapons and fighting styles are permitted for the duel, with few actual rules being applied to the fight itself.
This section concerns content related to Legion.
As the Burning Legion's third invasion raged and the city of Suramar was rocked first by a demonic invasion and then a large-scale rebellion, unrest among the nightborne populace began to grow. Nightborne nobles subsequently vied among themselves for an opening as an advisor to Grand Magistrix Elisande, ruler of Suramar. The noblewoman Ly'leth Lunastre, secretly in league with the Nightfallen rebels, attempted to gain the position to further the cause of the rebellion. With the aid of her allies, Ly'leth succeeded in gaining the required support and ousting the stiff competition.
As the ceremony to award the title began within the Sanctum of Order, with the Grand Magistrix herself in attendance, a rival claimant in the form of Coryn Stelleris leaped onto Ly'leth's platform and suddenly challenged her to Tal'ashar in front of those in attendance for the title. With no other choice, Ly'leth accepted, although she chose to name a champion to act in her stead. This champion, a disguised agent of the Nightfallen, clashed with Coryn Stelleris atop the Sanctum of Order. Eventually, Ly'leth's champion prevailed, slaying Coryn and securing the title for Lady Lunastre.
The origins of Tal'ashar are unclear but the ritual may predate the nightborne race, being inherited from the even older Kaldorei Empire. If so, this would make Tal'ashar many thousands of years old. No evidence for any such ritual is evident in modern kaldorei society, however, indicating that Tal'ashar is either a nightborne invention, or that it fell out of use among the night elves following the Great Sundering.
Regardless of whether it was referred to by the same name, it is highly likely the ancient kaldorei maintained a similar type of rite. Much of the aristocratic intrigue and powermongering that seems to lead to this ritual was directly inherited from that which notoriously existed in the Kaldorei Empire.