STORY SO FAR, if you want to binge only the latest arc, start with ch124 where shadow warriors was first introduced.
we are now in the empire special forces of magic (plus shadow warriors) vs serie's continental mage association featuring first class mages arc. motive is yet unknown but the empire is conspiring to assasinate serie in the grand ball during their national foundation festival. it's been established in earlier chapters of this arc that shadow warriors have been ordered by the empire to kill mages for a long time already (frieren is on their list) and not disclosing why.