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There is a condition in the substrate for the validators like if the validator misbehaves for example if the validator goes to offline than the stake of the validator will be slash, I setup a network of 5/5 and add one more validator and it goes to active state, after that i stopped the validator node and everything is working fine there is an event which show the offence of the offline validator but the stake of the validator not goes to slash 
So, how can I slash the validator stake
The default behavior is to only slash if more than 10% of the validators are offline (https://github.com/paritytech/substrate/blob/master/frame/im-online/src/lib.rs#L961-L971). The validator will be kicked out of the validator set in all cases.
Yep, there is a threshold as @André said.
But I think 1/5 is more than 10%.
There is one more thing that you need to know.
There is a SlashDeferDuration. Check your runtime configurations. The slash might not be executed immediately.
You could try querying the UnappliedSlashes from PolkadotApps.
