You and up to four willing creatures who link hands in a circle are transported to the Under World and you appear in the closest physically safe space that would hold your teleportation circle. If you are trying to teleport from a city where a hill was leveled to make room for buildings for instance, you would appear on top of the hill as opposed to inside of the hill.
Alternatively, if you know the sigil sequence of a teleportation circle in the Under World, this spell can take you to that circle. If the teleportation circle is too small to hold all the creatures you transported, the spell fails and you lose this spell slot until after a long rest.
You can use this spell to banish an unwilling creature to the Under World. Choose a creature within your reach and make a melee spell attack against it. On a hit, the creature must make a Charisma saving throw. If the creature fails this save, it is transported to the closest physically safe space in the Under World. A creature so transported must find its own way back to your current plane of existence.
* - (A dreamstone ring worth at least 500gp which is consumed in the process.)