Spells are the main way Avatars and other Spellcasters influence the realm. You cast minions, artifacts, auras, and magics. You can only cast a spell during your main phase, while the storyline is empty. (A few cards like Dodge Roll grant a timing exception.) When you cast a spell, follow this process: Choose the Spellcaster. This is often your Avatar, but it can be any Spellcaster you control. If your Spellcaster occupies multiple locations, choose the location where they're casting the spell. For example, River of Flame occupies two locations, surface and subsurface; when it casts a spell, you must choose if it's casting from the surface or subsurface location. Reveal the spell and confirm you have the necessary threshold and mana to cast it. Note: Both players are responsible for confirming that all spells are cast legally! The most common spellcasting mistake is having insufficient threshold. Make it a regular practice to check threshold on every spell that's cast! Choose additional details based on the type of spell. See below. Once you've done those three steps, you actually pay the cost to cast the spell and the card moves from where it was (usually your hand) and enters the storyline. It will then resolve according to the normal storyline rules.
To cast a minion spell, you must declare the minion's summoning location. By default, you must choose the surface of one of your sites. Some abilities may modify what summoning locations you may choose. For example, minions with Burrowing may be summoned directly to underground locations you control. The summoning location must be a legal location for that minion to enter at time of casting (regardless of what might happen after the spell is cast and enters the storyline). For example, you could not choose Gnome Hollows as the summoning location of an Autumn Unicorn.
To cast an artifact spell, you must declare the artifact's conjuring location. By default, you must choose the surface of one of your sites. If it's a carriable artifact, you may alternately choose one of your units and declare a location occupied by that unit; when the artifact enters the realm, it will be carried by that unit.
To cast an aura spell, you must declare the conjuring location(s). The aura's card text will often specify conjuring details, but by default, auras occupy 2x2 areas and can be conjured anywhere in the realm. Auras occupy the surface of sites and the void. To represent the aura's locations, place the card at the intersection of the squares it occupies.
To cast a magic spell, you must declare all targets of the magic. For example, if you cast Arc Lightning, you must declare the 'target nearby unit' at the moment you cast the magic spell. When the magic goes on the storyline, the target will have been chosen and can't change later when the magic resolves. Note that some magic spells do not use the word 'target' and instead use a word like 'ally.' For example, when you cast Overpower, you do not need to specify which ally you will choose, and instead you simply select the ally when the spell is resolving from the storyline. Finally, there are a few magic spells that don't target directly, and instead cause an ally to target. In that case, you also do not need to specify targets when you cast, since you only need to declare the targets that the magic itself is targeting. For example, if you cast Joust!, you do not need to declare the target when you cast the magic; you select the ally when the magic is resolving from the storyline, and then the ally (not the magic!) does the targeting at that time.