You gain the ability to meld with trees. When you touch a living tree that is your size or larger, you can spend 5 feet of movement to meld with the tree. While melded, the tree becomes an awakened tree that obeys your commands. You are physically subsumed within the tree, allowing you to see through its senses and use its actions, and granting you total cover within it. You can use your class features while melded with the tree, and spells you cast while melded treat the tree as the spell's point of origin, not you.
You can use your action to emerge from the tree, ending the melding and returning the tree to normal. Exiting a tree that you're melded with in this way is mentally taxing, and you must succeed on a DC 10 Constitution saving throw or lose concentration on this spell when you do so. If the tree is reduced to 0 hit points with you are melded with it, it explodes in a burst of magical flame. All creatures within 10 feet of the tree must make a Dexterity saving throw, taking 4d6 fire damage on a failed save or half as much on a successful one. You automatically fail this save.
At Higher Levels: When you cast this spell using a spell slot of 6th level or higher, you can choose to meld with a willing treant in addition to normal trees. While melded with a treant, your two consciousnesses work together to give you enhanced perception and reactivity. You have advantage on Wisdom (Perception) checks, Dexterity saving throws, and saving throws to avoid being stunned. Additionally, you gain an extra reaction that can only be used for opportunity attacks. If you use the Animate Trees action while melded with a treant, you can't use it again until you complete a long rest.
Not feylock?
Your "At Higher Levels" section should have a tooltip/link to the treant statblock.
This is so cool! Now I want to make a spell that does something similar but with elementals, like the investiture spells.
Does the treant have to be willing?
Wow. This spell would be very useful for fighting ranged enemies or extremely powerful melee opponents, such as a giant.
Ooh, good point. I'd probably rule it as it has to be willing, and not even have a saving throw. If it's not willing, you can't target it.
I've updated the spell to version 1.2, which now specifies that the treant must be willing. The intent of the spell is not to allow a creature to "possess" an unwilling treant.
Is this considered a domain spell for oath of ancients and nature domain?
Not by default, but it's totally within the DM's power to houserule it to be a domain spell. I don't think it breaks anything to swap it out with one of their other 3rd level domain spells.
If you are exiting the tree... the tree goes back to normal anyway, whether you fail or not. What does failing this do?
Can you enter the tree whenever you want if you maintain concentration?
That's correct.
I am becometh tree