Same as base game. Essences are substances taken from different creature parts, and each creature has different types of essences: air, earth, fire, water, and power essences. Enchantment requires different types and grades (faint, weak, glowing, radiant) of these essences.
Use a Mortar and Pestle (purchasable from Layla in the Grand Bazaar) on the creature parts to distill them into essences, on a stack of essences to combine them into a higher grade (two lower-grade = one higher-grade), or on a single essence to downgrade it.
Using the Mortar and Pestle requires an Alchemist's Workbench nearby, but there are somewhat-rare items that can split (rather than downgrade) and combine essences anywhere.
All poison recipes can be found in the Night Market in the Hive Ward. They all require a mortar and pestle alongside the Use Poison feat to make, but alchemists willing to spend the money can purchase an item granting this feat from Layla in the Grand Bazaar (as well as the mortar and pestle).
All poison recipes produce 10 doses. When applied to weapons poisons last for 10 rounds, +3 per level of Assassin and Blackguard.
For a list of poison recipes, see: Poison (item).
Brewing Potions Edit
Aside from the base game, Minerva sells recipes for more potent potions. All recipes require a number of ranks in the Craft (Alchemy) skill, the Brew Potion feat, an Empty Potion Bottle (purchasable from Layla), and a nearby Alchemist's Workbench.
For a list of potent potion recipes, see: Potions.
There are a number of bomb recipes beyond the base game. The available options can be viewed here: Bombs.
There are four items to help with alchemy that are found through random drops.
Shaper's Alembic: Divides essences without the need of an alchemy bench. It is particularly useful because it is more efficient, giving two lower essences rather than one.
Shaper's Retort: Combines two essences into one of a higher potency. While this can be performed at an alchemy station, both the alembic and retort work on entire stacks of essences at once.
Shaper's Decensor: This works on either an elemental or power essence. When used on two elemental essences, they are combined into a power essence of the same quality. When used on a power essence, it divides it into one essence of each elemental type, one step lower in quality. This item uses 5 charges per use and comes with 50 charges.
Magic Electrifier: While not strictly for alchemy, this is necessary for the Decensor. This item can destroy items to convert them into charges, then used on any item that uses charges to restore them. The number of charges given by destroying an item can be roughly determined by looking at the price offered by Trevor Longfellow. It is about 3900 gp in item value per charge. Note that all charges are expended from the electrifier on use, but items can only hold 50 charges.