Hello, I was wondering if there's a way to homebrew a familiar to add to the manage extra's tab. I have just gained a Tressym as a familiar in my campaign however, I don't see an option in the familiar or animal companion list to add this to my character sheet.

There isn't anything about the Tressym that is different from its standard state block and it will follow the variant rules for familiars. Any help would be appreciated.

You don’t need to “homebrew” anything, just add the Standard Tressym to your character sheet as a familiar:

Odd by chance is it only an option if you buy storm kings thunder as it doesn't come up as an option on my character's extra list.

Quote from DerekBacon >>

Odd by chance is it only an option if you buy storm kings thunder as it doesn't come up as an option on my character's extra list.

 
Or through Content Sharing (that’s how that👆example character can access it. You may be able to purchase just that monster however…. ☝️

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Yup:

Ah didn't know you could buy individual parts of the book. That would probably be easier and cheaper thank you.

Happy to help.