Concentration

Rules As Written

A caster can concentrate on one and only one spell at a time.

House Rule

• A caster with a proficiency bonus of at least +4 can concentrate on two spells at once.
• If a caster concentrates on multiple spells, he must make a concentration save immediately upon casting the new spell and again at the beginning of each of his turns. DC 10 + Total Level of Spells concentrating on. Each successful save increases the DC by 2. If the spellcaster fails this check, they lose concentration on the longest running spell.
• While concentrating on multiple spells, they are Slowed as per the spell.

Example: Shade casts haste, a 3rd level spell. Later, he casts banishment, a 4th level spell. He chooses to maintain concentration on both and is immediately slowed. He has to make a DC 17 concentration save. He makes it. Next turn, he continues to hold concentration and has a DC 19 save to make. He fails. He is no longer slowed and his haste spell ends. Banishment, being the only spell he is now concentrating on, is still in effect.