Slay

The English word slay means “to kill in a violent way”. It also figuratively means “to overwhelm” or “to amaze”. The past tense form is slew and the past participle form is slain. However, when used in the sense “to overwhelm”, slayed seems to be acceptable as the past tense and past participle.

Another English word slaughter can be considered as the noun form of slay (slay + -(t)er). We can see the similar formations in words laughter (laugh), fodder (feed), murder (Latin morī meaning “to die”), etc. Slaughter today can be used as a verb as well.

Lastly, onslaught must have something to do with slaughter, which means “a destructive attack”. This word is the combination of the prefix on- and slaught (obsolete English word meaning “slaughter”). The prefix on- here means “leading on to something” (e.g. oncost¹), so it totally makes sense that onslaught(attack) leads on to slaught(killing).