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For now they're only accessible with cheats- on the keyboard press '.' and then enter 'holditem' and after that, 'Poisoned Mouse'. 

In part 2 you'll be able to buy them without using cheats (seemingly from a rogue called Diesel going off of the content on the wiki).

I did not know how to access the cheat menu so I just changed my save file text and renamed my inventory item from Mouse, to Poisoned Mouse.

Also, to explain the spoillers, 

to get the exile ending, gift the poison to Cedarstar. In theory there must be more ways to get exiled, but I prefer the 100% completion when there is simply no more clan.