It is an Artifact Set called Immunity Set

It puts a Block Debuff Buff on a hero at the start of the fight which last 2 turns.

I use it on my M.Mage

You can see the picture below:

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It is often a very good Arena Strategy to use the Immunity set on your Cleansing champion.

This way if your team gets Crowd Controlled (Stun - Provoke - Frozen).

The Cleansing hero will not be affected due to the Block Debuff Buff.

The Cleansing hero will be able to remove the Crowd Control debuffs from the rest of your team. 

This way they will not skip their turn.

Awesome!  Thanks