New wither attacks:

-Skull fall: Summons a wither skull 10 blocks above 7 different targets that'll fall, so you have to build a roof or have to keep moving to avoid this attack, this attack happens every 4 seconds to avoid exploitations by putting the wither in a cave
-Side Skulls: wither skulls coming from the side, usually spawns 10 - 20 blocks away from their target
-Withering cloud: summons a cloud that inflicts wither 3
-Wither Skeleton Spawning: Summons wither skeletons to aid the wither in battle; can spawn with a sword or bow
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Items:

-Wither Chestplate: Has 9 defense(one point higher than a netherite chestplate); It gives you +5 extra hearts, +2 knockback resistance and +4 luck, Gives you immunity to wither. There is also a weaker version of this obtained before killing the Ender Dragon. The weaker version gives you 2 hearts instead of 5, 8 defense instead of 9 and no knockback resistance, luck and wither immunity - Obtained from killing the wither
-Arrow of Skulls: Summons wither skulls above the entity it hits or area that it landed - Obtained from killing the wither; use these wisely as they are one-time use and is not craftable
-Wither Sword: Deals 4.5 hearts of damage and have 2.1 attack speed (for comparison, all swords are 1.6), inflicts wither debuff. Also has a weaker version that deals 4 hearts of damage and have 1.8 attack speed. - Obtained from killing the wither.
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Advancements:

-A soul for souls: obtained from killing the wither, and entering hard mode
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Other:

-Wither will emit soul particles on death
-Wither can also drop wither roses on death
-Wither is weakened if spawned before the ender dragon is defeated and killing him during that, will give you a weak version of the wither chestplate
-Wither has 200 health and 6 defense before Ender Dragon and 300 health, 18 defense, and the new attacks listed above after Ender Dragon
-Wither Skeletons drop their skulls much more frequently
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New Mechanic: Hard Mode.

-Killing the wither will trigger hard mode
-There are two types of hard mode: Pre Ender Dragon, and post Ender Dragon
-During pre Ender Dragon hard mode, enemies' attack damage, both range and melee and follow range are doubled, and chances of them spawning with good gear is increased. Large slimes and phantoms can also spawn. All enemies defense are also set to a default of 6 instead of 0. Ender Dragon fireballs are also stronger and bigger. Zombies have a chance to spawn a deadly gas cloud on death. Illusioners teleport when hurt
-During post Ender Dragon hard mode, enemies' attack are increased 4 times(relative to Vanilla damage), and their defense are increased to 18. Ghast fireballs and creeper explosions are also twice as big and powerful. Pre Ender Dragon effects are still relevant though.