Talk:Excalibur/Abilities/@comment-23.16.89.136-20130726085558/@comment-4719392-20130726135541

It's a rare drop from enemies, usually infested. However you can't be missing the mod (or you shouldn't). You start with all 4 when you pick a warframe or when you craft one. Also, the game does not allow you to sell all mods of an ability (e.g. if you have 4 Slash Dash mods, you can only sell 3) if you have that warframe.