Nothing. They both do exactly the same thing, just different appearance and names.
In case you didn't know: They grant you the ability to find legendary Pokemon in certain floors of certain dungeons.
For example: If you have a Mystery Part/Secret Slab and go to Surrounded Sea, a Lugia will appear on the 18th floor.
However, you do not need to hold these items.
Hope this helps. ^^
You have to have the secret slab or the mystery part and bring it with you to miricle sea to 3F. (not deep)
If you have a Secret Slab or Mystery Part, Lugia will appear on floor 18 of the Surrounded Sea.
in miracle sea and other dungeons to recruit/fight Darkrai you'll also need a secret slab
Keep searching.
You might need to get a secret slab or mystery part first.
Yes, you neeb a secret slab or a mystery part to get legendaries in the final maze
You must have a Secret Slab/Mystery Part. He is in Dark Crater, 10F.
maybe you can get the secret slab and mystery part in your treasure bag
you get celebi in time & mewtwo in darkness but the only way you can get them is by having the secret slab or mystery part
You DO NOT USE the MYSTERY PART OR SECRET SLAB. You keep it in the bag for it to be able to do what it does.
Get a Secret Slab or Mystery Part, and go to Surrounded Sea B18F. Lugia's there, but ONLY with the Secret Slab or Mystery Part. but i went there twice and it was only there the first time
it's for recuiting darkrai and others
You first get a secret slab/mystery part (type wondermail genrator in google) and then go to Amp Plains (Make sure that the secret slab/mystery part is in your tresure bag)
For one its not mystery Slab its either myster part or secret slab and they let you meet legendarys in certain places like Zapdos in Amp Plains in the 7F.
Have secret slab/mystery part and then go to miracle sea 3rd floor
he is in amp pains if you have a secret slab and a mystery part
20 floor of midnight forest with secret slab or the mystery part You can also find Darkrai on BF3 in Miracle Sea after completing the whole storyline with the Secret Slab or Mystery Part in your bag (in versions Time and Darkness)