To descend on is to arrive somewhere suddenly and in numbers. Descend on the town.
It carries a hint of invasion whether the arrivals are welcome or not. Relatives descend on a household at Christmas in exactly the way the phrase suggests.
1phrasedescend onTo arrive somewhere suddenly and in numbers.
“Four thousand supporters would descend on the village every August.”
descend on is a phrase.
Practised in A Song of Ice and Fire