The Mallorn Ent is an Ent boss added in Public Beta 24 for evil-aligned players to fight. He is a stronger version of a normal Ent and follows the boss mechanics.
The Mallorn Ent can spawn with different names, suggesting that he is a dominant Ent rather than one character. He glows and does not attack people with positive Fangorn alignment, when not angered.
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 him in survival mode, you need to place a normal Mallorn sapling on a dirt, grass or podzol block and right-click it with a Gulduril crystal, creating a corrupt Mallorn sapling. You can also plant a corrupt sapling, if you manage to get it from somewhere else (you steal or buy it from another player on the server for example). You don't need negative Fangorn alignment to plant it, neither for the summoning itself. You should do this in or close to Fangorn forest, as you need some Ents nearby for the next step.
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 boss to spawn. Upon being summoned, he will say something like "Who dares challenge the might of Laurelindorenan!" (The original Sindarin word for Lothlòrien, the name later being shortened down.)
In creative mode, you can plant a corrupted sapling and use spawn eggs for the three Ents or 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
He can summon ents and huorns, and can also send a whirlwind of leaves towards you. He can jump considerably high, and heal by "absorbing" the essence of trees, so choose a clearing or chop down some trees before summoning him, to deprive him of this possibility.
After emptying its health bar for the first time he 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. Now, you've got to use fire as he seems immune to any pure physical damage.
Combat
However, to kill it, you'll need fire to weaken him and do significant damage, and it is recommended to use an axe during a fight instead of a sword, which even if it has more attack damage won't deal as much damage as it does the axe to him. The player should always bring something to set him on fire, as flint and steel, sulphur matches or lava, but the most useful items are enchanted weapons like flame on a bow, or fire aspect on an axe, with other enchants too to deal even more damage. Therefore the Mithril axe or battleaxe with fire aspect enchant (and sharpness) is the best weapon to fight the Mallorn Ent boss. You also should consider to bring some magical drinks to heal yourself and wear a decent armour.
Rewards
Upon death he 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.
If you have more than +1000 Fangorn alignment, you'll loose 1000 by killing him, otherwise your positive Fangorn alignment will be set to zero. If you've got negative alignment in advance, you'll loose another 50 points.