Whenever a magical hand is cast within range, you may choose to cast this spell as a reaction in order to summon a shimmering translucent hand of your own that will attempt to intercept and grasp it in an inescapable handshake.
If this spell is cast at a level equal to or greater than the triggering spell, then it is intercepted automatically. Otherwise both you and the caster must roll a d20 and add your respective spell level to each roll – the effect is intercepted if you roll equal to or higher than the caster.
A magical hand that is intercepted by this spell manifests as normal and lasts for its full duration, however its effects are totally nullified so long as the shaking hand grips it. While gripped, the triggering hand will only disappear once its duration expires, if concentration is forcibly broken or if it is purposefully dispelled using Dispel Magic or similar – its caster may not willingly choose to end it by other means. The shaking hand disappears when the shaken hand does.
* - when a magical hand is cast within range
This is a 5th edition reinterpretation of the old Halaster's Shaking Hand spell; the old spell was just Bigby's Hand with the bonus effect of being able to intercept other Bigby's Hands, but I wanted to make something a bit different, unusual and funny.
It's a very niche spell with niche applications, but the fact that it works with any magical hand should mean it can also effect Chill Touch, Mage Hand and a few other spells, and the fact that the shaken hand cannot be released voluntarily means you can potentially lock a caster into maintaining a spell that has become useless.