Regicide

Regicide is the act of assassinating a reigning monarch. This may be done lawfully (e.g. execution following a trial/overthrow), extra-lawfully (e.g. in battle) or unlawfully (e.g. hit-job).

Regicide can cause instability if there is no replacement for the monarch or his/her government, and also because the law has likely been violated.

Aeon of the Champion (AoC Universe)
Regicide, whether successful or merely attempted, is a re-occurring theme in the Aeon of the Champion universe.
 * During the events of Antiquity, Archangel Lucithel committed 'regicide' by killing the titans of the Pantheon (AoC Universe). Note that the titans never professed royalty, but could be considered rulers in that sense.
 * Surtyr, under the demonic influence of Lord Tiz'Galaath, duelling, humiliating, and then decapitating Samsonus of the Boneclaw tribe, which gave him control over all Jotunheim.
 * Lord Shen'Gan and Tiberian's uprising against the King Smurf, Dan'Ariz, to prevent him from using the Cerulean Plateau as a point-of-contact for the summoning of Sargamon.
 * The other dragon aspects rebelling against Dragonqueen Tiamat, leader of the black dragonflight during Mythos, after they found out that she had betrayed them to the Chaos Gods and their monstrous masters.
 * Darkhorn's uprising against the Goldhearts (the Unicorn Kingdom's royal family) during Middle Age, which gave him control of High Faerun but deprived magic of one of its greatest guardians at a critical moment.