Ok ill look into it... this will be tricky if you don't know much about World Editor
Copy and paste in custom buffs, the Buff Unholy Aura (object editor > buffs/effects > undead) and rename it "Morph" (near the bottom of the list of things) and rename the extended tooltip "This unit can morph" (the very bottom of the list)
Copy and paste in custom abilities, the ability Unholy aura and edit the stats of movement increase and hp regen to 0 then rename it "Morphing ability" and then find "stats - levels" then make the levels 1, then find "Level 1 - buff" then change that to the buff "Morph" that you made.
Make an item (Object editor > Items > Right click on custom items at bottom of list > create new item) in creating a new item, find the icon of "unholy aura" - the item name is helm of battle thirst then put the name up on the top something like "Helm of Morphing" then delete all the ability's of the helm then add "Morph" ability.
There's one problem with the next part, is there going to be a certain unit getting an item or multiple units? You will need to do some triggers if its multiple units.
(For single unit)Now, copy the ability "Destroyer Form" (Object editor > Abilities) Change the name to "Morph", change the icon to what you want. Change the "Normal form unit" to whichever the unit holding the item is, change alternate form unit to the unit you want it to turn into and then delete all morphing flags.
Create a unit that the ability lets it morph into, then make another ability, edit the same things to the opposite way round so it can morph back to the first unit.
If it is for multiple units i would not know how to do that, well, i do know how to do it but it will contain variables that can leak, sorry... you will need to ask someone else.
To give unit the ability when it gets picked up:
New Trigger
New event > Unit > Generic Unit Event > Unit acquires item
New condition > Boolean Comparison > ((Triggering unit) has buff Morph) equal to True
New action > Unit > Add ability > Add Morph to (Triggering unit)
To turn it off when unit drops:
New Trigger
New event > Unit > Generic Unit Event > Unit drops item
New condition > Boolean Comparison ((Triggering unit) has buff Morph) equal to False
New action > Unit > Remove ability > Remove "Morph" from (triggering unit)
Uh, got a bit lazy at the end
i wouldn't have needed to HAVE SO MUCH if you knew a DECENT AMOUNT of stuff about WORLD EDITOR
*Falls to sleep*
I may as well turn this in to a tutorial now...