Found a cool chest the other day, didn't think to take a screen cap but it was in Balzy...
The chest was called "Sword in the Stone chest" or something like that, it's the first time I've gotten one and it dropped a mythic - nothing else - just the mythic.
Are these mythic-only chests?
Has anyone else found one?

Yup, very rare to get it spawn in a chest spot. It 100% chance drops a fighter weapon, and is called Mythic sword in a stone. You also got a non-rb version of sword in a stone.

I didn't get any Mythic from these till now.
What a good Mythic-Spot is, for me at least, are the "Great Weapon Rack"s
These often Spawn RB's for me

Fighter weapon?
No love for mages/rangers then?

Yeah, I got a purple chain sword earlier from that chest type. I assume there are ranger/mage versions, if not at Balz than at some boss...though it would be nice if all 3 classes were represented. I haven't paid close enough attention to notice.

Nop, this refers a bit to Excaliber, no love for rangers and mages, YET.

The great weapon rack dropping rainbows is new to me.. I think I have to help updating the chest page on TT, didnt even realised that lol.

Yup i saw at least 3 rb swords in stone aka excalibur lol

Got the Excalibur reference, but I was wondering if maybe the devs went out on a limb and made something like 'Staff in a Stone' or maybe 'Arrow in a Stone'.
Also the mythic I got was from a non-mythic chest... at least I think it was.

I found Not the sword of gosh on my first "Sword in the Stone chest." Only found it once out of hundreds of boss runs.

Yup, I just had one drop an Axiom. First time I've had more than one RB in a session, so I'm very happy. (Along with Axiom, two purples, on normal. Woot!)

Ironic, and grats.

saw it once, gave me a purple sword.

I've done some farming recently, i've got rbs out of the weapon racks, i think it was just a normal rack. I've got rbs from the shrines or whatever, the statues, I've got a few rbs from the boss chests, once in frumps and once in verigrims, frump gave me a crap ring, and grim chest gave me his helmet.

Haven't seen the mythic sword in the stone yet, but every normal sword in the stone drops unique fighter weapons.

On a only slightly related note, i got a triple drop from verigrim to, that was awsome 3 rbs all from grim himself, 2 were non mem, so that was extra cool.

last night i got a soulbound rb ring off the warrior kings coin vender up near the bank. I didnt know they could make RB gear there.

Yep, they aren't very common, but you can get rbs there. Although I don't suggest you just start dumping coins into it. In testing it on the pts this last go on pts, i pumped over 900 kcs into it and only came out with 1 rb.