ROTOM

We all know TV rots your brain, but who ever suspected that programming could physically attack you? Rotom is an Electric/Ghost pokemon that lives in a TV set in the mansion near the exit of Eterna Forest. While Rotom's a bit too weak to truly be considered a Legendary Pokemon, it's still only obtainable through this sidequest, and cannot be bred.

Unlike the other ghosts that inhabit the manor, Rotom is only broadcast during prime time. Try visiting after 8:00 PM - that way, you'll be sure it's on the air. Head into the small room with the TV aglow and examine hit. Give the old box a good thump to bring Rotom out for a fight.

Rotom is only Level 15, but is quite tough to catch if its HP is high. Since your regular team can all KO it in a single hit, you'll probably want to bring along a lower-level pokemon to effectively chip away at its life force. Get it down into the orange or red zone, then start throwing your poke balls at it. Since it's night, Dusk Balls are probably your best bet here. It shouldn't be too long before Rotom succumbs to the demands of his viewership.