You can find the Mod Rod (also known as the Staff of Transform) at the end of the Royal Crypt, to the south of Endor. Besides having some fun uses, you’ll need this item to progress the story.

Preparations

• Your party should be around level 23. After you get the Zenithian Shield you should be around that level. If you’re not, grind around Rosehill until you’re there.
• You should have defeated Balzack already. Beating him will give you the Flute of Revelation that you need to get into Rose’s chamber.
• Have the Ultimate Key. You’ll need it to enter the crypt. You can get it by Helping the queen of Femiscyra.
• Update your gear on Rosehill. The Rosehill vendor has some expensive but worthy weapons. The Falcon Knife Earrings on Alena can make a lot of difference, as she has a natural high chance of critical hits, and a weapon that attacks twice per turn doubles that chance.

Finding the Royal Crypt

After you reach Rosehill, use the Flute of Revelation and defeat the guard, Rose will ask you to stop Psaro. The Slime in that room will tell you that there is a Mod Rod on a crypt to the south of Endor that you can use to fool the monsters in the Diabolic Hall.

Exit Rosehill now and use Zoom to teleport to Endor. Get into your ship and follow the river to the southeast. On the tip of the southern peninsula you’ll see a building that you can only reach by ship; that’s the crypt. Get inside.

Exploring the Royal Crypt

If you remember the cave where you reencountered Alena, then this place will look familiar, as it has the same sliding floor puzzles; they are a bit simpler this time, though. 

Floor 1 to BF 2

After you enter, walk north to get to the next floor. Step into the northern sliding tiles to reach the next set of stairs. From here pick the only path available to reach yet another set of stairs. 

BF 3

Step on the sliding tiles in front of you. Open the chest to get a Staff of Antimagic. You can use this staff as an item to cast Fizzle on monsters, so give it to someone who doesn’t know the spell.

Turn west from here and then walk south, turn east on the bifurcation and follow the sliding tiles but without touching them, to find a Mini Medal.

Now walk south until the end of the room and then turn east, past the sliding tiles, and turn northeast on the next bifurcation. You’ll find the stairs to the next floor there.

BF 2

Head west and open the room with a healing circle that will refill the HP and MP of your entire party. You can use it unlimited times

Walk south to reach the center of this floor. Walk around the sliding tiles without touching them.  Now go through the southern path and turn west on the next bifurcation to reach a room with your next set of stairs.

BF 1

You’ll start beside the first sliding tiles you took. Avoid touching them. Head straight west to get into a room that is a straight pathway. Exit through the southern door and keep heading south, ignoring the sliding tiles again. To the right you’ll see a room with a chest, you’ll find the Mod Rod there. 

If you already got everything in this dungeon, cast Evac or touch the sliding tiles to the north of the room to get back to the start.

Congratulations! The Mod Rod is yours now. You can use this item to transform your entire party. Sometimes they’ll become random humans, and sometimes they’ll become random monsters. 

This item’s main use is to turn into a monster to speak with the other monsters in the Diabolic Hall, but you can use it in Rosehill and some other places to trigger alternate dialogues.