Celebrant of the Triune is an uber monster located in Toraja which are immune to everything until attacked by Edyrem used from the Summon Edyrem skill, which drop their shield and enable them to be attacked.
AR means Attack Rating.
Attack 1 AR
Attack 2 AR
Regen Rate tells what % of a monster's hit points are recovered per second.
Blocked attacks will not damage the monster. Monsters need not carry a shield to block.
Avoided attacks will not damage the monster.
Drain Effectiveness tells what % of a character's mana and life steal apply to that monster. 100 is all, 0 is none.
The chill effectiveness on almost all monsters is -20%, regardless the difficulty.
Resistances over 99% are listed as immunities and monsters will take no damage from that type of attack.
Immunities can be "broken" by skills that lower resistance, if the value drops below 99%. See the Resistances page for more details.