Tome of Order is a Legendary magic weapon obtainable from any mob (except Statues) with a 1:25000 drop chance.
Shoots energy bolts which stick to enemies and tiles
They explode after some time or can be blown up manually, by pressing RMB, dealing extra damage.
It is a Legendary magic weapon, which means that its stats will grow with every boss killed, including some of the modded ones, for example, bosses from Calamity mod.
Please note, that killing same bosses over and over again will not upgrade the stats of this weapon.