well no one gave you a complete long answer so i guess i can explain the phrase.
it comes form warhammer 40k. the whole phrase is "i feel the warp overtake me... it is a good pain" and is generally used by chaos psykers who are opening their minds to the warp. a psyker is a individual with a strong spiritual connection to the immaterium, a realm of souls that has been corrupted beyond saving by the darker emotions and thoughts of all sentient races. it is festered with daemons and nasties who want to wreak havoc in the material realm for giggles. back to psykers, they can use their minds and bodies as gateways between the warp and the physical realm and harness it's energies to preform psychic feats. this however is very dangerous and attracts demons since the psyker is basically opening a window between the warp and the material realm. strong will and diciplin is therefor needed from those who want to use it without to many side-effects. chaos psykers however worship the warp and it's daemons and use a less refined technique. by intentionally using the more more violent energies of the warp they can function more like a hose to blast warp energies out the ass but as said not without consequences. those may include mutations, insanity, stroke, cardiac arrest, brain death, daemonic possession and violently exploding into chunky salsa from trying to channel to much power.
one thing that is constant though is the pain of having the energies of the warp and raw stuff of chaos claw at your mind, soul and body.
thus the expression "i feel the warp overtake me" (the warp energies gathering to be unleashed) "it is a good pain" (the pleasure of power and the fulfillment of serving their gods.)