This spell prepares a vial of water to accept another spell, allowing the caster to make an "instant" and temporary potion without going through the steps normally associated with making one.
The spell's material component is a vial containing at least one ounce of water; the water must be reasonably clean but need not be pure. The caster must touch the vial of water while casting Darsson's potion. Immediately afterward, the wizard (or another spellcaster) must cast another spell, directed at the vial of water, using all that spell's normal components. Rather than having its normal effect, this second spell is absorbed into the water, creating a temporary potion that can be imbibed any time during the duration of the Darsson's potion spell. The second spell then takes effect on the imbiber as if the spell itself were cast on the imbiber (but never as if the imbiber were the caster).
The spell cast into Darsson's potion must be 3rd level or lower in strength and must be one the caster could and would cast on himself or herself. For example, feather fall, shield, or cure light wounds could be cast into Darsson's potion because they affect the caster directly. Spells like friends, identify, shocking grasp or fireball cannot be cast into Darsson's potion because those are spells that the caster would always use to affect an object or another person. If a question arises, the DM decides whether or not the spell functions.
The secondary spell stored in Darsson's potion has standard effects according to the level and ability of the spellcaster who placed it into the elixir.