.. Mausoleum door

The key is clearly a sigil of the family.

Look around at the garden. Who was the garden for?

It commemorates Eustece Magtheridon.

Does the name ring a bell?

You've met someone from this family, already.

Go talk to Tybalt Magtheridon, at Le Boule de Poils.

You have to use the indent, first, to get Tybalt to give you the key: their family medallion.