I'm after some cool ideas for a Legendary Greatsword I'm creating for my son. He's a lv.8 Gold Dragonborn Cleric who serves Diancecht (god of medicine and healing) and he's about to multiclass as a Paladin. Talona is being a bit of a madam in our campaign, so the idea of Diancecht calling upon him as a holy warrior, fits nicely. He's about to receive a legendary greatsword but I need some ideas of powers it could have, that fit with the above. Based on something else I read and taking influence from Storm Sphere, I was thinking as an action the sword is plunged into the ground creating a 20ft radiant sphere, each enemy in the sphere when it appears or that ends its turn there must succeed on a Constitution saving throw or take 2d6 radiant damage. As a Bonus Action, you can zap an enemy within 60ft for 2d6 radiant or zap an ally who regains 1d6 hit points. As an action, you can draw the sword from the ground to end the effect.

I'm after any and all suggestions, from massively OP to ultra sensible and any bizarreness in-between :D 

That sounds like a very reasonable idea and certainly unique enough for an item. If I could make a couple of suggestions though:

• You should probably specify that he has to remain in the sphere for it to work. Otherwise, he can stab the sword into the ground, take out another weapon and just lock down a huge area on the battlefield. Also, you should probably give it a similar time limit to the sphere since it is otherwise an unlimited amount of healing that could also be used out of combat which would mean that the moment combat ends he can just heal people to full for the price of nothing.
• Think about maybe limiting how many times a day he can use this feature. It is pretty good, so he should probably not be able to use it indefinitely since that opens it up to a lot of abuse, even if done accidentally.
• Regarding the zaps, make sure to specify that the range is measured from the centre of the sphere, that's important!
• Also, think about either slightly bumping up the damage of the sphere itself, maybe to 2d8 or 3d6 - Which should be safe since Divine Smite cannot be added to area of effect abilities (Thank the lord) - or possibly adding an "on a successful save they take half damage" clause since a lot of monsters have a very high Constitution saving throw bonus - Especially when compared to Storm Sphere's Strength saving throw - and the sphere does not come with the same rider effects as the spell - The difficult terrain bonus whilst within it, as well as the disadvantage on checks to listen to something.
• Think about how you rule the attacking zap. What kind of attack does it count as? This is important because it decides whether or not Divine Smite can affect it.
• You should also be safe bumping the healing zap a little, maybe up to 1d8 or 2d6. Since both of them are a bonus action so he can only do one of them per turn it should balance out decently.

Those are some thoughts I had reading the item over. Which of them you follow, if at all, is up to you in the end. The idea of the items seems really cool, honestly. Especially as it is based on a rather obscure spell that I personally had never heard off.

Anyways, have a nice day and happy Roleplaying!

I would also recommend giving the sword a couple of minor magical properties related to medicine and healing. For example:

purify food and drink 1/LR

Emits a faint glow in the presence of a poison (within 15 feet of a poison)

advantage, flat bonus, or proficiency with medicine checks while attuned

If the sword is specifically intended for use by a paladin, you could consider tying it to paladin abilities.

For example, while the blade is planted in the ground, if the bearer is within 10 feet of the blade, as a bonus action they can use Divine Smite or Lay On Hands on a target within 60 feet.

Being able to use normally melee/touch abilities at range as a bonus action is still a pretty big deal while being naturally limited.

If you want to keep the damage aura relatively simple, you could make it a once per long rest free casting of Spirit Guardians that doesn't require concentration when you plant the sword, lasting as long as it remains planted. This means you don't need to worry about defining the aura mechanically as the spell does that for you (and is one a paladin might already have anyway so should be familiar).

So by planting the sword you're getting a free uninterruptible (except by pulling the sword) spell as part of that action, while boosting the range and versatility of your paladin abilities so long as you stay near it. This isn't as powerful as some of the other suggestions but it should still be strong enough to count as legendary, depends on the power level of your campaign really.