WHAT / WHY
One of the gripes I have with D&D 5th edition in general is that combat is a little simplistic, and there are few truly dangerous encounters in mainline PC games. After having played Pathfinder Wotr, I wanted the devs to put in a few hidden bosses like they did In DOS2, that are designed to make you want to rip out your own eye sockets. The Timekeeper for me at least, could be a foe that would present such a challenge. Killing the entity would give you a usable *Great Father Clock* [allowing you to do an extra long rest every 24 hours].
Where Would it Be?
in Dark Alliance where you confront Zarad in the infused clock tower. The player could visit this area somewhere around act 2.
Immunities / stats + how to beat it
All standard immunities / immunities , the timekeeper broke into the encampment once you killed Zarad and devoured his body. As such this timekeeper has a AC of 28 [17 Nat + 3 mage armor + Zarad].
* to cheese the boss, you must find a left hand, and pickpocket the boss from stealth DC20, and put it in, doing so instant kills the boss.