The Mallorn Ent is a Ent boss added in Public Beta 24 for evil-aligned players to fight. It is a stronger version of a normal Ent and follows the boss mechanics.
The Mallorn Ent can spawn with different names, suggesting that it is a dominant Ent rather than one character. It glows and does not attack people with positive Fangorn alignment.
Defeating the Mallorn Ent will earn you the achievement "Wrath of the Forest" and also give you the title "Entslayer" and the Entslayer's Shield.
Summoning
To summon it in survival mode, you need to place a normal Mallorn sapling on a dirt, grass or podzol block in Fangorn Forest and right-click it with a Gulduril Crystal, creating a corrupt Mallorn sapling.
Then, some Ents will appear next to it and try to cure the corrupted sapling; three Ents have to be killed in order for the Mallorn Ent to spawn. Upon being summoned, he will say "Who dares challenge the might of Laurelindorenan!"
In creative mode, the boss can be summoned using the command:
/lotr_summon lotr.MallornEnt
Once you summon it, a purple health bar similar to the Ender Dragon's appears at the top of the screen.
Abilities
It can summon ents and huorns, and can also send a whirlwind of leaves towards you. It can jump considerably high, and heal by "absorbing" the essence of trees.
After emptying its health bar for the first time it will say a phrase, saying something like:
"O Kementari, hear me! Strengthen me against the iron-bite of this little orc!"
(Kementari is a name for Yavanna, the creator of the ents, her more common name may also be used)
It will then commence to glow green and gain what seems to be infinite health.
However, to kill it, you'll need fire (lava or through a flint and steel) to damage it significantly. An axe can do less damage, but is more effective than a sword. About halfway through the second health bar, the boss weakens and hits become increasingly effective.
Rewards
Upon death it drops around half a stack each of Mallorn sticks and Mallorn logs, a Charred Mallorn Mace (50% chance), and a Mallorn Ent Trunk as a trophy.