You can resurrect and recruit Sir Mervyn for your party at the top of The Tallest Tower, a dungeon located on Wetlock Island. You can do this part of the story after you get the Flying Carpet.
You should try and find him as soon as you reach this point in the story, to make sure he’s not very underleveled compared to the rest of your party.
Warning: this guide has some spoilers for Dragon Quest VII.
Finding the Sizzling Stone
As soon as you get the Flying Carpet, a ghost inside Tombes des Héros Inconnus, to the east of La Bravoure, will tell you about a magic stone you’ll need to revive a legendary hero.
Zoom to Estard and talk to Pike, your hero’s uncle, at the tavern. He’ll tell you he sold the stone to a rich collector, Bulgio. You can find clues about his location by visiting El Ciclo and Gröndal.
After doing that, Zoom to L’Arca and use your Flying Carpet to cross the lake to the north and enter the mansion, Palazzio di Bulgio.
Talk to Bulgio in the treasure room and ask for the stone. He won’t give it to you, but after exiting the mansion, he’ll change his mind and join your party, giving you the Sizzling Stone.
Climbing the Tallest Tower
Recommended level: 28
Zoom to Gröndal and fly on the Flying Carpet to the south to the nearest island. You’ll see the Tallest Tower on the northwestern part of the island.
Take the path to the right and follow it until you reach the middle of the room, then turn north and take the stairs.
On the second floor, follow the main path to the south to reach the next stairs.
Walk east and north. Be careful not to step on the southern edges of the room, or you’ll fall off the tower. follow that path and you’ll reach the next floor.
The fourth floor is just a long stair. Climb it to reach the first labyrinth of the tower.
There are no chests on this labyrinth. The easiest way to cross this floor is to take the path east, and circle the entire floor through the outer rim, until you reach the southern side, then turn north on the first bifurcation and follow that path to the next floor.
The sixth floor is another room with just a large winding staircase.
The seventh floor is another maze. Take the path south and then east, turn north after the second bifurcation and follow that path until you find the stairs on the southeastern corner.
You’ll see four elevators going up. Pick the second one to reach the last maze.
On the ninth floor, walk through the outer rim to the right, enter the path to your left and follow it all the way south, then west and north, to reach the top of the tower.
Recruiting Sir Mervyn
Once you reach the top, talk to Bulgio and pick the first choice, then walk to the altar in the middle and use the Sizzling Stone.
After answering some questions to Sir Mervyn, he’ll join your party! You can exit the tower now, going to the previous floor and falling off the edge.
Sir Mervyn starts at level 19, which is probably much lower than the rest of your party at this point, so you’ll have to grind some levels for him first. His stats are fairly balanced, but he’s slightly better at magic than physical attacks, so choosing a magic vocation for him could be a good idea.