Astral prevents to reroll to zero, you reroll to 4 if the enchantment fails. Immortal prevents that your item is broken during the alchemy process.
If you want to level up an item at higher level you also need steady on it. When alchemy fails above +5 a red mark will sometimes appear with a loss of durability. That loss can go to 99% so your item is totally useless. That red mark can't be removed with stones or rerolling characteristics, it's permanent. The only thing that you can do is improve the durability white mark on the item to compensate your loss.
And a couple of lucky marks are also welcome, they enhance the probability that your alchemy will be successfull.
So if you want to +++++ an item, yes you need astral and immortal. And no, you can't find that stuff in game, it's just available through item mall.
