PoE 3.16 changes: it is no longer a Notable ascendancy passive skill. Now, give everyone access to Versatile Combatant as a Keystone Passive.
-25% to maximum Chance to Block Attack Damage
-25% to maximum Chance to Block Spell Damage
+2% Chance to Block Spell Damage for each 1% Overcapped Chance to Block Attack Damage
(Overcapped Chance to Block is the amount by which Uncapped Chance to Block is higher than Maximum Chance to Block)
(Uncapped Chance to Block is the value Chance to Block would have if ignoring Maximum Chance to Block)
This means that if you have a 75% chance to Block Attack Damage, allocating this Keystone Passive grants you a 50% chance to Block Attack Damage, as well as a 50% chance to Block Spell Damage.
Flavour text: To master the arena, you must be prepared for anything.