I don't think it goes against the spirit of the ability at all.
Its not like its impossible to fight an invisible foe. So the warlock issues a divine challenge, then does his best to vanish from view, moving around his opponent in circles, zapping him with Eyebite, and taunting his foe as his foe swings his sword ineffectively.
Sounds good to me.
Its not like its impossible to fight an invisible foe. So the warlock issues a divine challenge, then does his best to vanish from view, moving around his opponent in circles, zapping him with Eyebite, and taunting his foe as his foe swings his sword ineffectively.
Sounds good to me.