If this is the same Magnolia O&G in the website link below, they are the real deal and active players in the trend.
The 18.75% Royalty is a low ball offer, in my opinion. 25% is more the norm today - especially in established areas like most of Karnes County.
However, every area is different as to subsurface conditions and "prospectivity). Take a look a the presentations on their website and determine where in Karnes County your minerals are located.