Heart Slates

A legendary Pokémon's Heart Slate is an item that may be dropped by legendary bosses in dungeons. When a Heart Slate is combined with a Mystery Part by Merlin, the Part can be used to summon the Heart Slate's legendary in a dungeon, which lasts for the duration you are in the dungeon.

Location and Rarity
Heart Slates are dropped by legendary bosses in dungeons. Legendaries in harder and longer dungeons typically have a higher chance of a drop. In contrast, legendary Pokémon in easier to access locations have a lower chance of drop rate. Legendary Pokémon such as Regice that spawn as floor bosses drop heart slates very rarely.

Combining Heart Slates with a Mystery Part
Merlin the Alakazam combines Heart Slates with a Mystery Part is located in the Southeastern Village of Archford. He resides in the Wisdom Hub on a hill on the eastern side of the Village. To fuse a Heart Slate with a Mystery Part you must have the following: * The cost of fusing a Heart Slate with a Mystery Part can be found by the formula:
 * Master Rank or above (13,500+ Explorer Points)
 * Heart Slate
 * Mystery Part
 * 100,000 Poké, increasing with more slates*

(n + 1) * 100,000 Poké

n is equal to the number of Heart Slates already fused with your Mystery Part. For example, if you already have 3 Heart Slates combined with your Mystery Part, the formula for the cost of the fourth will be:

(3 + 1) * 100,000 = 400,000 Poké

Summoning
Bring the Mystery Part into a dungeon and have an empty slot in your party. Use the Mystery Part and you will be able to select which legend you would like to summon out of the ones you have collected. Upon summoning the desired legend the Mystery Part will automatically be sent back to Kangaskhan Storage. The summoned legend will last for the duration of the dungeon and will leave upon exiting or escaping.

Trivia

 * You will be able to save millions of Poké if you get a new Mystery Part for every legendary Heart Slate you find.