Instant Death
noun, verbal noun, の adjective
An instant death is an instant death.
そくし
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
彼は潮津波に乗ってサーフィンをしようとしたが、残念ながら即死した。
He tried to ride tidal bore and surf it, but unfortunately he died instantly.