You gain Oath Spells at the Paladin levels listed.
3 Hail of Thorns, Ensnaring Strike
5 Locate Animals or Plants, Healing Spirit
9 Conjure Animals, Spirit Guardians
13 Grasping Vine, Polymorph
17 Tree Stride, Insect Plague
When you take this oath at 3rd level, you gain the following Channel Divinity Options:
- Life Sequester:
Over the course of ten minutes, you transform a small or smaller non-magical object, or a willing creature you are touching into a beast of CR 1/8 or lower, expending a use of channel divinity. The character's armor and equipment is absorbed into it while it is transformed, and the creature gains no benefit from its armor and equipment. The character retains the intelligence, wisdom, and charisma scores it had before being transformed. The character or object returns to their original form after 8 hours or after being reduced to 0 hit-points. If you transform an object with this feature, the beast it turns into is neutral towards you.
- Golden Knight:
As an action, you use your Channel Divinity to summon an Animated Armor that appears in an unoccupied space within 10 feet of you. The Animated Armor vanishes if it is reduced to 0 hitpoints or if it goes more than 10 feet away from you, or after one minute. The Animated Armor is friendly to you and your allies and obeys your commands.
Aura of healing:
At level 7, whenever you cast a paladin spell, you can choose a humanoid ally within 30 feet of you. They regain hitpoints equal to the level of the spell you cast plus your proficiency bonus.
At level 15, whenever a creature does damage to a creature you have summoned with a paladin spell or Channel Divinity, you can use your reaction to make the attacker take magical bludgeoning damage equal to the damage it dealt (to a maximum of 40 damage). Once you use this feature you cannot do so until you take a long rest. Additionally, when you fail a saving throw to maintain concentration on a spell, you can use your channel divinity to attempt the saving throw again instead (you must accept the results of this new roll).
Field of Futility:
At level 20, as an action you undergo a transformation that lasts for one minute. While under this transformation you gain the following benefits:
- You can cast counterspell as a 3rd level spell at will without expending a spell slot.
- When you hit a creature with a critical hit, that creature takes an additional 4d10 necrotic and 4d10 psychic damage.
- You are immune to radiant and necrotic damage.
Once you use this transformation, you cannot use it again until you complete a long rest.
In terms of balance
Most of this subclass seems fine, although a few of the features I'd probably buff a little bit, if only because they're otherwise quite difficult to use.
- Channel Divinity: Golden Knight. I would revise the wording to "The Animated Armor vanishes if it is reduced to 0 hitpoints, if it ends its turn more than 10 feet away from you, or after one minute.". Otherwise, it requires very finicky movement tricks to move around the battlefield.
- Alternatively, if the intent is for the Animated Armor to be leashed to a specific point in space, I would just rewrite it as "if it ends its turn more than 10 feet away from where it was summoned". Either of these would be a lot better for the overall behavior of the creature.
- Superior Summons. There's really no good reason to cap the damage at 40 damage. There's already plenty of features in the game that deal retributive damage to a target—see, for example, the Oath of Redemption's Channel Divinity: Rebuke the Violent, which at level 3 allows the paladin to do exactly this, without a damage cap, once per use of Channel Divinity, with the only mitigating factor being a Wisdom Save for half damage instead—and there's already at least one feature that allows a PC to immediately deal lethal damage to any creature, mitigated only by a single saving throw—that would be the Way of the Open Hand's Quivering Palm, which costs only 3 Ki points (itself a resource that recovers on a Short Rest). So since this feature is already limited by being usable once-per-long-rest, I don't think there's any need to further limit its capabilities in this way.
- Superior Summons, Additional Effect. I would just make this limited by one use per short rest. Tying this feature to the Channel Divinity feature essentially means you're giving the Paladin 3 Channel Divinity options, breaking the common design convention of only giving a Paladin (or Cleric) 2 Channel Divinity options, and being able to reroll a single concentration save is probably never going to be worth sacrificing your Channel Divinity (or, more likely, withholding their Channel Divinity explicitly for its use), itself already core to most of the rest of the class features.
- Animated Armor. Given that this creature is going to be summoned at all levels of play, it might make sense to give it the capacity to scale with the paladin's level. I recommend using the Artificer's Homonculus Servant (Eberron: RftLW, pg. 62) as a template for homebrewing a creature that could be summoned in the place of an Animated Armor (you'll probably have to dial the hit points back a bit, maybe more like 3 hit points per level instead of 5).
In terms of Theme
A lot of those balance issues are pretty minor, in my opinion, and easy to deal with. The bigger issue with this homebrew, in my opinion, is that it's a little bit scattershot in terms of the gameplay style it facilitates. The subclass is called the Oath of the Golden Knight, and its Channel Divinity features facilitate summoning creatures, but its Oath Spells list seems to instead be split between nature-themed spells having to do with control (Ensnaring Strike, Grasping Vine) or summoning, and its level 7 feature (ostensibly replacing the "Aura" most paladin Oaths grant at this level) instead promotes their healing ability. There's a bit of a mismatch between what the features are permitting this Paladin to actually do, and what this Paladin Oath is intended to do.
I would consider revisiting both these features, thinking about a different level 7 feature that would better fit with the other features granted by this class, and thinking about reconsidering the spell list granted to this character, perhaps to focus more on buff spells—spells like Bless or Mantle of the Crusader would be especially appropriate for this class.