。
Bees attacked without mercy.
Literal
Bees [subject-が] mercilessly attacked-came
容赦なく is a stock adverbial meaning 'without mercy, mercilessly' — 容赦 = mercy, clemency; なく is the adverbial negative ('without'). 襲ってくる uses ~てくる to mark the motion as coming toward the speaker — intensifying the sense of being targeted.