The Seal of the Orichalcos

You spend ten minutes placing nearly invisible runes upon the ground in the pattern of the Orichalcos and designate a single creature by name as a target and up to 6 creatures as allies to the Seal. A creature can use its action to inspect the runes and make an wisdom (perception) check against your spell save DC. If it succeeds, it becomes aware of the runes upon the ground.

If the target steps within the centre of the seal the following effects activate: 

• No-one besides the allies to the seal can exit it via mundane or magical means.
• Any teleportation magic used within the seal targeting outside of it automatically fails.
• Allies of the seal become Life-bonded until the target is dead or the seal is dispelled.

Life-bonded:

• Any damage taken by a member of the allies is divided by their total members before resistances are taken into account, rounding to the nearest whole number even if that number is 0.
• When a creature takes damage via Life-bond they do not make concentration checks unless they are the one to take the damage.