Like any other skill in The Sims 4, you have multiple ways to earn levels in Thanatology. If you plan on pursuing a Reaper career, then it'll be necessary to start studying for the sake of promotions.

The Undertaker may not be an active career, however, it does branch out to two different branches (Mortician and Funeral Director) as Sim gets promoted to level 6. With that, there is a bit of choice as to which skills Sim may want to improve,

The Reaper career in The Sims 4 Life and Death brings a whole new perspective to the life of the Grim Reaper, as you become their protégé. Starting out as a Grimtern and working your way up to a fully-fledged Reaper isn't going to be easy, but thankfully you can head along to work with your Sim to help guide them on their way.

When your Sim ventures into one of the crypts around Ravenwood in The Sims 4: Life & Death, you'll find any number of odd items inside.