I can't speak to the subclass yet - I'm waiting until the next patch to do my own break down of the classes and such, but to answer the question at the bottom, in terms of core rules, maintaining concentration is a constitution saving throw (the DC is 10, or half the damage you took, whichever is high). Concentration is a lot more shady in BG3, however, since a number of things inflict "incapacitated" that shouldn't... and incapacitation immediately breaks concentration, without a save. Getting knocked prone, or getting frightened, for example, will both automatically break your concentration when neither should be doing so (This thread:
https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=739348&page=1 has a write up on how BG3 has implemented status conditions, compared to the core rules, and has a section that discusses the impact this has on concentration).
