Oof, 6/6, haste for 4! In Limited, Hellbent is pretty easy to set up. Gorehorn Minotaurs was very scary in M12, and arguably more difficult to set up.

By the time you've emptied your hand in Limited, though, it'll be some way past turn 4. And making your Hill Giants more relevant in the late game seems perfectly reasonable. This would be a jolly good card in Limited, because on turn 4-5 it's a 3/3 and on turn 8+ it's gigantic, but I don't see why that's a problem. It'll be very rare for this to get 6/6 on turn 5.

Mmm, you have to be going faster than "Draw, land, spell" to get it that fast. By no means impossible to get an accelerated early game, and indeed quite nasty when you do.

But still, it's one creature. It's removable, or chumpable (or even boltable during your draw step?) and the cost of keeping it huge is having no fallback position at all.

Seems good, but not hideously "Run 4 no matter what" good; to me.

Well, I think would run as many as I got in Sealed pool or Draft. Even if I don't have hellbent on turn 4, I'll still play a Hill Gaint. And on turns 6/7+ I will very happily slam a 6/6 with haste for only 4 mana.

Red just doesn't usually get creatures that big. Eventually, in basically every Limited game, your Hill Giant will upgrade into some giant Green/Blue size monster. With Hellbent, this will often be the biggest dude on the field. For 6 power in red, you might have to get Flameborn Viron. And 6 toughness is almost unheard of in red commons. The card might be all right if it gave +3/+0 instead of +3/+3. That way, late game you can slam it for some surprise damage (6!), but it doesn't dominate the board.

Right now this just looks so much better than, say Blade-Tribe Berserkers and Shatterskull Giant.

And yes, it's just one creature. But it's common, and common's are not supposed to be any where near "answer me in a few turns or you lose". If you use a removal spell on this guy, I may very well have 2 or 3 more, and then at least you can't use it on any of my rares. Again, this is all in Limited.

This seems about has hard to accomplish as Blade-Tribe Berserkers. With the Berserkers, it was more about threshold of artifacts in your deck and how fast you could get them out there... which could be tough, or easy, depending on how the draft went. With this guy, it's all about "How many fast, yet potent cards did I draft", which seems about the same to me.

That being said, the Beserkers only got the bonus when they etbed, then the +3/+3 went away. I'm not going to say whether Deepcave is too powerful or not, because I'm not sure. I'm pretty sure hubatish is right that this card is first-pickable, and worth building your deck around. Personally, I like to leave cards like that in the uncommon slot... not because they are powerful, so much so than because I don't want every draft in red to revolve around a card like this. Gorehorn Minotaur seemed fine, since the point behind Bloothirst in 2012 was to find a way to get damage in early, and support with Bloodthirst. That was red's strategy. If this card stuck, I would say that red's strategy in Labyrinth draft would be to "dump my hand at first possible convenience without caring what I put on the table assuming I drafted 3 Deepcave Giants". Hellbent doesn't have to work like that, though that could work, if that's what you're going for. Is that the response you want?