Okay, so first of all let me answer the obvious/not-obvious question "Why is Hell Axe the only Equipment - Attack card in the game??" Well, to be perfectly honest there just isn't a lot that Equipment can do to make Attacks more interesting. The main thing an Equipment can do is make the attack repeatable by letting you do it every time the Defender attacks. You could do a one-time effect when the Equipment enters play or when the Defender dies but that's really just tacking something on without using the Equipment card type to add anything.
BUT, there is room for an Equipment with a one-time effect that changes based on the Defender's stats. Basing anything on another card's stats is sticky territory though, and that gets us into why I started with this design! The reason Hell Axe set the Defender's attack value to 3 is because otherwise it could scale way too well with certain Defenders. Put it on a Knight and you'll maybe get 6 damage out of it. Put it on a Whispers of Legend and you could easily get 24 damage out of it for the same 3 energy cost!!! AHHH!!!
This is a design lesson- having abilities that directly scale off an attribute from another unknown card can get messy fast. You'll often have some card out there that is fairly costed but has that particular attribute hiked all the way up, and suddenly you have a deadly combo that randomly ruins games. This is why Hell Axe had to set the Defender's attack value to 3. Letting it get boosted by other Equipment generally produced predictable results since there are only 11 Equipment cards so it wasn't something I was worried about.
After Some Playtesting
Here are some notes I made after a playtest with this version:
Hell Axe - This card was pretty dang good, I think it dealt like 9 damage for 4 energy? But that's a pretty conditional 9 damage. Gonna put this one on the watch list but I'm not really confident in raising its cost just yet. Spending 5 for almost no damage increase would hurt. Also I've got to remember I used Heal to get extra mileage out of it sooo maybe it is fine...
So yeah, in a game I managed to deal 9 damage with this. However it was bolstered by the fact I had a card called Heal that made Defenders live longer. (I removed Heal from the game when I realized there were like 6 too-similar low cost cards that made Defenders live longer.) So anyways this wasn't quite the "perfect storm" I thought it might be- on basically any big Defender or with any Defender-healing card it seemed Hell Axe could do 6 or 9 or 12 damage too easily. Doing 6 damage for 4 energy is fine if you have to work for it, but when that 6 damage turns to 9 or 12 with a little more work things start to get bad. It scaled too greatly when you enhanced it with other cards.
Segway time- obviously Hell Axe was RIDICULOUS with Giant Turtle which is a 1/10. It made the turtle a 3/10 and since the Turtle lived forever, it was virtually guaranteed to be able to do 9+ damage. I distinctly knew when I made Giant Turtle that it would close off some potential card designs. For example, imagine a card that costs 4 energy that says "Discard a Defender to deal damage equal to its health to each opponent." Most of the time this would be mediocre or okay but with Giant Turtle, 4 energy for 10 damage is way too good. BUT, I knew that this would close off some design space, but not a lot, so I was fine with it. I haven't found any cards I've been super excited about that it inhibited, so I am 500% happy about creating Giant Turtle.
So How Did I Fix That
Okay here's the next (and final) version of Hell Axe:
WUT? So now it costs one less energy, but always makes the Defender a little worse and always deals a flat 2 damage (whereas before it essentially dealt a flat 3 damage). Why did I make these changes?
Okay, so first of all I said part of the problem with the original version was the scaling. You had to work to deal 6 damage. But if you could attack again, it went up to 9. Then up to 12. By lowering the damage per turn to 2 it scaled way slower obviously. By the time you're attacking three times you've only dealt 6 damage instead of 9, then dealing 8 damage instead of 12 is still really awesome for three energy but not like ridiculously overpowered like 12 was.
So quick note- the whole "dealing damage equal to the attack value" and still getting enhanced by equipment was all a bunch of unneeded jargon for something that dealt a flat amount of damage 99% of the time. That's why it just says "deal 2 damage to the opponent" now.
The -1/-0 was mostly a jab at Giant Turtle (and it helps it not to be overpowered with other cards like Medium which is a 1/6). You can't have Hell Axe go on a Giant Turtle and deal 10 damage to the opponent while Giant Turtle keeps hitting your monsters. This nerf sounds ridiculous, but it's actually a laser strike to the root of the issue. The only cards that can have astronomical health are ones with 1 attack, because by making it 2 attack you've doubled their damage. And if you can keep something alive forever with a Hell Axe on it, you can't have it dealing its full damage and have Hell Axe going off because it's too much scaling. So overall the -1/-0 does exactly what we need it to.
So How DO You Use Hell Axe (STRATEGY TIME)
First of all, recognize that Hell Axe is a tool that makes games end quickly by letting you invest resources into directly hurting the opponent. If you're going for a long-game prize card or gold strategy then Hell Axe isn't going to help you very much, but if your opponent is you'll be able to make the game end faster. Cards that help you in the short term like Lost at Sea, Murder, Black Blade of Death, Hire Mercenaries, and Brocky Noble Mercenary are all great. If you stay ahead long enough to kill your opponent then it doesn't matter that you blew all your resources.
Furthermore, opponents that are investing for the long-term with cards like Infinite Sleep or Death Ray will probably be a little behind, and that's when you Hell Axe them. If they're trying to fill your decks with Confusions you also probably want to Hell Axe them.
Okay, if you want to get full use out of Hell Axe you'll have to be CAREFUL. Sure, it's called Hell Axe but that doesn't mean you just attack recklessly. Your goal is to attack as many times as possible, and take the least amount of damage each time you attack. Generally you're aiming to take 2 damage when you attack. If you can attack a Hellhound which will rarely happen, that's even better.
If you just throw Hell Axe on a Knight, you'll likely get 4 damage out of it and lower the Knight's total damage output by 2. Spending 3 energy and a 2 damage reduction for 4 damage to the opponent is not optimal, but in some games any kind of pressure on the opponent is deadly to them.
Recognize when a Hell Axe strategy is not the way to go, and use its Equipment special ability to pay 2 energy and discard it to draw a card.
COMBOS. Hell Axe on a Giant Turtle can easily deal 10 damage to the opponent for a 6 energy investment on your part. Look out for other high-health Defenders like Royal Guard or Medium. Iron Armor (+0/+4) is a good boost and can often add an extra 4 damage to the opponent for a 1 energy investment. Healing Flask (heals a Defender to full health at once or 1 damage/turn) can be aaaaweeesome with a high-health Defender. Giant Turtle + Healing Flask + Hell Axe is pretty much a godly combo that your opponent probably won't let you draft, but would come close to doubling the damage Hell Axe deals.
SO, bottom line- Hell Axe is a tool and not a blunt object to toss on any Defender and throw around. It lets you be the one to decide how long or short the game is, and that ability is deceptively powerful.