Д228
В ДОВЕРШЕНИЕ (K ДОВЕРШЕНИЮ, ДЛЯ ДОВЕРШЕНИЯ ) чего
PrepPthese forms onlythe resulting
PrepP is sent
advas a final addition to (the circumstances, events, features etc mentioned in the preceding context and usu. summarized by the complement of the idiom)
to crown (cap, complete, add to) sth.
в довершение (к довершению) всего - to top it (all) off
on top of everything elseon top of all this (that)
в довершение несчастья (беды и т. п.) - to make things (matters) worse.