#showtooltip
/castsequence reset=5 Dark Transformation, Apocalypse
On the first press of the button/keybind, it will cast Dark Transformation. On the second press of the button/keybind it will cast Apocalypse. If the button/keybind is not pressed for five seconds, it will reset back to Dark Transformation.
You can remove the reset=5 option if you want as it is optional.