massacre
殱 centers on large-scale, violent destruction: a massacre that wipes out many lives at once.
殱 is the modern Japanese simplified form of 殲. The older form combines 歹 (death) with 韯, which likely contributed the sound and the idea of complete destruction. The modern form retains the same components but with a slightly simplified right side.
The left side 歹 means death, and the right side 韯 looks like a complex weapon or a pile of bodies. Picture a scene of total destruction where death is everywhere.
For セン, imagine a thousand (千, せん) people being massacred: 千 -> セン, and the death radical 歹 marks the tragedy.