What document do I need to serve a previous homeowner that is still occupying a house I bought at auction? I need the right one for the state of Maryland. The house is already in my name.

You need to find a landlord/tenant attorney in the area.  There's more to the eviction process than just a single document.  In some state, homeowners have some rights even after the auction.

As Jon said, it’s a process, not a document. Either some form of an ejectment order or an eviction. Find a local attorney.

Or you could go over an introduce yourself ( have a friend with you ) Or send a letter .. Explain that you bought the house at auction , and explain to them they need to packup and move . Maybe offer some cash ONCE THEY ARE OUT .  Also explain to them if you have to file eviction , that most landlords wont rent to them because an eviction . Dont expect to be welcomed with open arms , but be nice since they can trash the house and you cant do a thing about it