The first trick to remembering all of the options is to realise that in the potion creation stage, nearly all of the choices are fundamentally the same no matter whether the spell costs 1, 5, or 10 mana. The cost of the spell simply powers up the effect. There are ten different effects to choose from, which are outlined below. The spells are remembered more easily when you group the effects by numbers.
1, 2, 3 effects:
Icecap: Freezes 1, 2, or 3 enemy minions.
Ichor of Undeath: Summons 1, 2, or 3 friendly minions that died this game.
Kingsblood: Draws 1, 2, or 3 cards.
Shadow Oil: Adds 1, 2, or 3 random Demons to your hand.
Other effects in number order:
Felbloom: Deals 2, 4, or 6 damage to all minions.
Goldthorn: Gives your minions 2, 4, or 6 extra health.
Netherbloom: Summons a 2/2, 5/5, or 8/8 demon.
Heart of Fire: Deals 3, 5, or 8 damage.
Stonescale Oil: Gives 4, 7, or 10 Armor.
Mystic Wool: This card is unique in that it has no effect for 1 mana. It either turns a random enemy minion into a 1/1 Sheep, or all enemy minions into 1/1 Sheep.
At the moment, many of the pairings work according to the game engine, rather than in the best order, which means that Kazakus is not working as intended. An example of this would be minions that are getting killed by damage are then getting frozen before leaving play. This is correct in terms of how the game works, as deaths are always processed last, but the spells are going to be made to work outside of the regular game engine. This will make them more powerful and intuitive.
Mike DonaisWe are already working on this. Feel free to post stuff like this on our bug forums. (source)
This reply on Reddit confirms that the idea of the custom Spell is that it will resolve in an order beneficial to the caster, and that it will likely be changed very soon.