Bacon Bits
Hero
Yeah, the immovable thing is weird. It might be better to say, 'cannot be wielded with proficiency' or something. Essentially, it's totally unwieldly without attunement.
I would just say it's too heavy to wield as a weapon. If you want to make it difficult to move, require a DC 15 Strength (Athletics) check to lift it if you're not attuned.
I don't see how it does lots of damage? There's only one damaging property and that's the 10 foot AoE. Which only does 2d6+5 with no chance of a crit because it's a dex save for half. Which could be a lot of damage if he's totally surrounded, if you add it all up together. Unless you are just talking about wielding a 2d6 weapon in one hand as a lot of damage. Once again, in my estimation, the 'monkey grip' seemed the ability most prone for abuse.
The one other ability just knocks people prone. which is a lesser form of Gust of Wind, I think(if I'm remembering the correct spell).
I'm not saying it does too much damage or that you should tone it down. The only thing I think is actually a problem is the "can't lift it" part, and you changed that.
I'm saying that it deals a lot of damage as it's purpose. That's what it does. It's a big hittin' stick.
That's perfectly fine. But it's still a one-handed weapon with 10' reach that deals base 2d6 damage. That's above par on two degrees and you gotta acknowledge that. It has an ability that you get up to 6 times in one day that deals 2d6+5 (i.e., slightly less than 4d6) damage to 24 squares. That's a lot of AoE damage for a melee weapon. Again, that's fine if it's what you want, but it's still big. If there are 3 or more targets, you're already above par for what martials are normally limited to.
And it has another ability that lets you knock half a dozen people prone. That's not direct damage, but indirectly it certainly is because it's an offensive debuff. It allows your party to get advantage on melee attacks. It prevents escape. If you can find a way to use it against flying creatures (i.e., you activate it while on a flying creature) it's devastating. That's fine, but it's what's going on.
That's perfectly fine if that's what you want. A Flame Tongue Greatsword also deals a lot of damage. 2d6 base plus 2d6 fire. That's fine, but you have to acknowledge that that is what it's doing.
I don't think you need to change anything with the abilities. You just need to know that it's capable of a lot of damage. When you give it out, you just need to know that it's going to be powerful like a Flame Tongue, Rod of Lordly Might, Dwarven Thrower, or Holy Avenger.
It's a powerful item, and I'd be inclined to save it for the last half to last third of a campaign like it were a +2 or +3 sword. I would not expect that I would be giving out any weapons that were clearly better.
Attunement: to attune to this weapon, you must sacrifice a magical weapon which the sword absorbs into itself
I don't think this is necessary. Maybe if you wanted it to be an artifact or a relic, but I don't think it's that necessary. I would only use this if you wanted to prevent PCs from swapping the item around. I don't think that's really a concern. You can do it if you want, but it does nothing to balance the item. It's either worth the cost or it's not.
Here's my flat judgement:
- I think reach is fine. I kind of like the idea that you've got to use it two-handed to benefit from it, but that's not a big deal. Threatening reach is different in 5e so it's not really a balance issue.
- As I said above, I don't like the idea of a one-handed weapon having the Heavy property or benefiting from GWM. That's just weird, but that's also just me. I don't think it's wrong or anything, I just don't like the feel of it, if that makes sense.
- I think 2d6 damage from a one handed weapon is fine. It's like base 1d8+3 damage, which is high but not obscene. It's par with Frost Brand.
- I think call to hand can be free, just like the shrinking and growing. It's cool, but unless there's really something you can do with it it's not necessary to limit it. After all, Bladelocks and Eldritch Knights already get this.
- I think shared charges are a bit much. I prefer fixed uses per day simply because the bookkeeping is much easier.
- I think the AoE is pretty high damage, and it's what you're really attuning the item for. I'd probably set it to 2/day and then adjust later as needed.
- I think the knock prone cone is fine. I'd probably set it to 3/day. It's not too powerful, but it could be obnoxious if they spam it. The PCs should be able to get the most benefit from the first use of the power, and limiting uses encourages more strategic use.