Heart Slates

A Legendary Pokémon's Heart Slate is an item that may be dropped by Legendaries in dungeons. While players cannot recruit Legendary Pokémon (in the sense like most Pokémon are on PMU) and use them in town, 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. While Mystery Parts are not droppable, Heart Slates are, and are therefore tradable.

Location and Rarity
Heart Slates are a rare drop by Legendaries in dungeons, as a boss or a Secret Room spawn. 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 can spawn multiple times drop heart slates very rarely.

Combining Heart Slates with a Mystery Part
In the Southeastern Village in Archford, accessible after defeating Boggy Wastes, there is an Alakazam named Merlin that combines Heart Slates with a Mystery Part and resides in the Wisdom Hub on a hill on the eastern side of the Village. The requirements needed for the fusion of a Heart Slate are:
 * A Mystery Part, found at the end of Tanren Chambers, Normal Route, at Floor 99
 * A desired Heart Slate
 * Master Rank or above (13,500+ Explorer Points)
 * 100,000 Poké, increasing with more slates

The cost of fusing a Heart Slate with a Mystery Part can be found by the formula:

$$(n + 1) * 100,000 Poké $$

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 Legendary you would like to summon out of the ones you have fused into your Mystery Part. The Legendary summoned will be the level of the highest-leveled Pokémon already on your team. Upon summoning the desired Legendary, the Mystery Part will automatically be sent back to Kangaskhan Storage. The summoned Legendary will have a random set of moves possible for it, but you can also teach it a TM move for that specific instance

Obtainable Heart Slates
The following table lists all available Legendaries with Heart Slates.

Trivia

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