The Gengarite, Gengar’s Mega Stone, can be found in the custodian’s cottage at the Battle Resort. The custodian’s cottage is on the northeastern part of the island, and his cottage also contains a variety of other items you can get for free.
But the Battle Resort area is only accessible after completing the Delta Episode.
Gengarite Location (Step-by-Step)
After completing the Delta Episode, Norman in Littleroot Town will give you the S.S. Ticket. You can then board the S.S. Tidal and travel to the Battle Resort using this ticket.
When you first arrive, you’ll be greeted by a couple of familiar faces you’ve met throughout your playthrough. At this point you should be able to explore the Battle Resort, and you can get your Gengarite.
Step 1: Start outside the Battle Resort’s Pokémon Center. Begin heading east from the Battle Resort’s Pokémon Center.
Step 2: Continue east, past the Day-Care Center.
Step 3: Once you’re past the white picket fence, turn and go north.
If it’s your first time here, a brief cutscene involving Looker will begin to play.
Step 4: In the Battle Resort, the camera automatically rotates as you travel along the beach.
But just follow the same road, traveling east alongside the ocean after the camera rotates.
Step 5: Proceed east and look for some palm trees and cottages.
Step 6: The camera should rotate once again at this point.
Just keep going down towards the water’s edge, then go into the nearby cottage (pictured below).
Step 7: To obtain the Gengarite, pick up the sparkling item sitting on the north side of the room.
Is Mega Gengar Worth Using?
Mega Gengar is one of the most feared threats in the game thanks to his insanely high Special Attack, high Speed stat, and—most importantly— a near-exclusive Shadow Tag ability. It can use Shadow Tag to selectively eliminate foes it wants to trap.
So if you can build it right, yes this Mega Evolved form is definitely worth using.
When Gengar Mega Evolves into Mega Gengar, its ability changes to Shadow Tag, preventing the opposing Pokémon from switching out.
Note: Ghost-types and Pokémon holding a Shed Shell on the other hand are immune to this ability and can still switch out.
Mega Gengar can also use utility moves like Taunt and Destiny Bond to threaten a wide range of opponents.
This Pokémon is also capable of using the dreaded Perish Trap: a strategy where you use Perish Song and the ability Shadow Tag together to trap a foe & make the opposing Pokémon faint in three turns.
Mega Gengar can stall the countdown to Perish Song by using Substitute and Protect. Then on the final turn, you’d switch out to only make your opponent faint.