Magic is its own tool, and spellcasters can approach their casting skills with workmanlike precision. Being able to manipulate what your spells do is key to casters being able to do what they want even if requires a bit of manipulation of how the magic usually works.
Staff of Spell Girding
Staff, very rare (attunement to a spellcaster required)
This slender, fluted staff is surmounted by a large, faceted jewel, and it reinforces the power of your spells to withstand effects that might end them. As long as you are holding the staff, spellcasting ability checks made to use dispel magic to end a beneficial spell affecting you have disadvantage, and you have advantage on Constitution saving throws made to maintain concentration on spells.
In addition, this staff has 10 charges. While holding it, you can use the staff’s power in several ways by expending 1 charge.
· Extension. As a bonus action when casting a spell, you increase the spell’s maximum duration by one step, from 1 round to 2 rounds, 1 minute to 10 minutes, 10 minutes to 1 hour, and 1 hour to 8 hours, or doubling the duration for any spell with a duration of 4 hours or more. This has no effect on spells with an instantaneous or permanent duration, and spells that require concentration still end if you cease concentration.
· Linger. When a spell you have cast would be dispelled, as a reaction you can expend 1 of the staff’s charges to cause the spell to last for an additional 1d4 rounds (up to a maximum of the spell’s normal duration).
· Refocus. When a spell you have cast would be ended by breaking your concentration, as a reaction you can automatically succeed on your Constitution saving throw to maintain concentration.
The staff regains 1d6 + 4 expended charges daily at dawn. If you expend the last charge, roll a d20. On a 1, the jewel atop the staff crumbles into powder and the rest of the staff corrodes into nothingness.