You need to complete one the Imp's endings:
Fight him a lot and when you can "please him/suck him off"
When given prompt, submit to him
Go to Nyx's hideout and talk with the demon
Return to the Imp and tell him of the encounter
Go back to the hideout and you will fight the imp
To get the Word of Power achievement (as well as another one) you need to lose the fight (if you win you get a different achievement) and then in another playthrough (be reborn) fight the Imp again, but this time "press him against your body" (dominant option) and then after a few encounters he will submit to you and you can tell him his name (altough this option appears to lead to a softlock, meaning that it's unachievable)