The hydra also goes infinite with any sacrifice outlet. Removal is a necessary function of the game, and there is no need to make a creature that is utterly immune to it. Indestructible and Shroud are as irremovable as a creature ought be, since they get around specific forms but leave room for specific answers. BTW, Sudden Spoiling isn't in many envrionments so what would standard have to do about it? You have an 8 cost sorcery to get an infinite number of infinite sized creatures using something as simple as Vampire Aristocrat.
An ETB dual land needs some text sure, but I don't think the discard-to-play thing is a good move. Discard doesn't need to be hosed, especially when the strong discard cards would avoid it. You only hose "choose and discard" this way, sometimes random discard too but low odds so why put this in a deck?
And I can't wait until you tell me what I'm just waiting to hear, cause man, do I ever have a massive rebuttal for that one, and I'm just praying you let me use it.
You sound like a troll just now. Answer me honestly, would you want to see Stone Elemental in a pack? Would you want to build a deck including it? Lastly, do you feel it is fertile design space?
Stone Elemental is like Force of Savagery, useless without combo and begging for some kind of combo to break it. This can't work out well, either you have a wasted card or you make it viable enough that it's stupid good. The middle ground is so small and easily missed if it even exists so I suggest scrapping the idea.
10 lands that all hose discard make a more powerful statement than one creature. This is not an ability that needs to be on 10 lands unless you violently hate discard as part of your concept for a set.
ETB dual okay. But if your opponent has any discard tech, you turn that into card advantage. What this says is that you want to make an environment with no discard whatsoever. 100% unnecessary.
Text versions are easier to quote. Anyway, about the first one you can save some text a few ways. Lord is a useless type that was removed, so you can cut that. Given the cost, I think you can simplify the abilities into "BG: Regenerate target creature." Honestly, I think you could go further than that, since 7cc 6/5 regeneration could do without four colored mana and being uneven. It could easily cost 5BG, or less, and could afford to do something more.
Any land that can accelerate on turn two is impossible to judge without extensive play testing. I wouldn't mind seeing a fetchland variant that had one Island or Plains ability and also some other ability that fetched both, but that would also be impossible to guess the balance casually.
So it makes white or blue mana only to be spent to white or blue spells. And this is a limitation because it can't... help me out. I mean, they interfere with the next card, but white and blue mana is intened to be used to cast white or blue spells generally so this is a pretty insignificant drawback.
For the double fetch, turn two getting three land, two of which are untapped, is way better than normal fetches. So you exile it, that's something but let's say you only run three colors here and decide to run 8 instead of 12 since I"m not sure if you include enemies, this lets any color get 3 land turn 2 regularly and 5 land on turn 3 often enough. I don't see why everyone wouldn't use these.
Keeping the green one at +3/+3? At sorcery speed +3/+3 isn't as impressive to be sure, but referring to Immersturm makes me assume there are reusable blink effects so this could be a problem. Maybe a +1/+1 counter would be better, since worst case a 2G 3/3 ain't bad.
2R for a 2/2 and 2 player damage is suspect. Ghitu Slinger had echo, but on the other hand sucked. I'm not sure where we are now for such things. At 1 damage I'd be more confident, since it can still kill a creature. That said, I find it harder to cite precedent that any custom card is overpowered. I don't think anyone playing your set is going to scream bloody murder for 2 damage to a creature. You might get some irritated at a Goblin Hero +2 to the face as a common.
Edit - you can further simplify Dust-Shrouders by saying "When ~ enters the battlefield, another target creature" instead of using two instances of Cardname. I believe they do the same thing.
I think then the white and red abilities should be optional, or at the very least not be forced to target themselves. I can't think of a good reason to do this of the top of my head, but the white one can just stay basically phased out for as long it targets itself. I'm not saying Soul Warden life gain would be disruptive, but this can be avoided by the inclusion of one word on a card that isn't pressed for space.
The white one plays well into the rebel/mercenary style. It would be a shame to not play into that, but making it a rebel could impact its balance. The black one could be a mercenary then, and it would be better to pull out at instant speed (say during your opponent's draw step when you have no cards in hand) but mercenaries were never on level with rebels. If you want to avoid these interactions, the white one could easily become a wizard, cleric, or something like that.
Is black the problem right now? Anyway, this would just make black not use discard. This isn't metagame so much as killing design space.
You sound like a troll just now. Answer me honestly, would you want to see Stone Elemental in a pack? Would you want to build a deck including it? Lastly, do you feel it is fertile design space?
For the double fetch, turn two getting three land, two of which are untapped, is way better than normal fetches. So you exile it, that's something but let's say you only run three colors here and decide to run 8 instead of 12 since I"m not sure if you include enemies, this lets any color get 3 land turn 2 regularly and 5 land on turn 3 often enough. I don't see why everyone wouldn't use these.
Edit - you can further simplify Dust-Shrouders by saying "When ~ enters the battlefield, another target creature" instead of using two instances of Cardname. I believe they do the same thing.