Hello, Rufus!
Let me try to explain how it will looks like (for me). I don`t know how hard it will be to create such system, so please do not hate me
Let`s start
I think about content rewards or even skills. But I like you idea with code. Let`s try to mix them, your idea and mine:
1) Complete the achievement, get the code for +50 Gold (for example). Some codes will give you 10 Gold (easy one). Some will give 100 or even 200.
2) In Dark Depths location, somewhere in tha tunnels labyrinth, you can find hidden Store. Where you can buy an additional Skin for your hero. Like zombie slayer or something else
But the cost of this Skin will be more than 1000+ Gold. It means you need to complete a lot of challenges to get these codes.
Or 2+ Skins with a different cost. From 1 000 to 10 000
Or even new weapon in Shalis hands, just a funny model maybe.
I want to say that you can sell content stuff as a bonuses. Any you want. Beautiful auras, Wings and etc.
3) Create more challanges that will depends on these content stuff from #2. Like:
Kill the final boss with Toy Hammer (Toy Hammer in Shalis hands: long range, fast speed, much longer than usual, but only 1-2-3 damage per hit).
Such achievements will give more Gold. Or, As I said before, they can give you a special skills. For example you can activate abitily to see invisible units and objects. With this ability you can find Hidden NPC who will give additional quest(s) or just a hidden store where you can buy epic stuff.
So, you can add some buffs, bonuses or even debuffs (like Toy Hammer) to these content stuff.
The End!
You can start hate me
Oh, I forgot to tell you about the "bad" part of this story:
All these codes will be used by other players, who found them in "Google". So, I think, you need to make them unique for each player.
Or... just forget about it
Make a game for honest players 
Let me try to explain how it will looks like (for me). I don`t know how hard it will be to create such system, so please do not hate me
I don`t think that such kind of bonuses will be usefull. We are talking about interesting and hard challenges, but such bonuses as "Damage, Health etc" will make game easier.A problem I am having is rewards though.
The only rewards I have currently, is bonus stats (plus Damage, Health, Regeneration, DMGX), and a few ability upgrades (such as faster dashes, lower cd on stun). However, those ability upgrades needs to go at specific moments in the game, so the only thing I can reward for such achivements as clearing a map in master mode, is extra stats.
Perhaps though, I could make it so you can play bonus maps if you manage master mode or something.
If you have ideas for rewards, let me know!![]()
Let`s start
I think about content rewards or even skills. But I like you idea with code. Let`s try to mix them, your idea and mine:
1) Complete the achievement, get the code for +50 Gold (for example). Some codes will give you 10 Gold (easy one). Some will give 100 or even 200.
2) In Dark Depths location, somewhere in tha tunnels labyrinth, you can find hidden Store. Where you can buy an additional Skin for your hero. Like zombie slayer or something else

But the cost of this Skin will be more than 1000+ Gold. It means you need to complete a lot of challenges to get these codes.
Or 2+ Skins with a different cost. From 1 000 to 10 000

Or even new weapon in Shalis hands, just a funny model maybe.
I want to say that you can sell content stuff as a bonuses. Any you want. Beautiful auras, Wings and etc.
3) Create more challanges that will depends on these content stuff from #2. Like:
Kill the final boss with Toy Hammer (Toy Hammer in Shalis hands: long range, fast speed, much longer than usual, but only 1-2-3 damage per hit).
Such achievements will give more Gold. Or, As I said before, they can give you a special skills. For example you can activate abitily to see invisible units and objects. With this ability you can find Hidden NPC who will give additional quest(s) or just a hidden store where you can buy epic stuff.
So, you can add some buffs, bonuses or even debuffs (like Toy Hammer) to these content stuff.
The End!
You can start hate me

Oh, I forgot to tell you about the "bad" part of this story:
All these codes will be used by other players, who found them in "Google". So, I think, you need to make them unique for each player.
Or... just forget about it
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