Strain casting

When you’ve run out of spell points or you’ve cast your 6th to 9th level spells, you can attempt to cast another spell.

To do so choose a spell to cast and after you cast the spell make a Strain Check which is a spell-casting ability check where the DC is 10+(2x spells level).

Failing the check means you can’t cast that spell or if you upcast it, a spell from the level you cast at, until you finish a long rest, unless it’s a 1st level spell cast at 1st level. This casting strain does not apply to cantrips.
Every time you cast a spell and pass the strain check, the DC increases by 5 for that spell.

(Optional Strain Damage)

If you fail the Strain Check DC make a CON SAVE with the same DC, on a fail you take 2x the spells level in necrotic damage. This damage can't be reduced or prevented in any way.