There are plenty of spells that require you to think creatively if you’re going to make the most of them, but only one of them makes you think with portals.
Arcane Gate is essentially the Portal Gun in spell form. It lets you create a pair of circular gates that hover in mid-air, and what enters one instantly exits from the other. They can float freely in space and are only visible from one side
As an aside, this raises the question of what it looks like when someone enters a portal facing away from you. Do you have to watch in horror as the front half of their body appears to melt away into the ether?
Body-horror aside, this really is an excellent spell that can be incredibly useful both in and out of battle. The only real downside is that it requires a sixth-level spellslot and so has to compete with several other immensely powerful abilities.
Depending on the circumstances, however, a well-placed Arcane Gate can allow your party to deal more damage than a Chain Lightning, Circle of Death or other combat spells. If the enemy commander is hiding behind a legion of minions, for example, it can allow the Barbarian and Paladin to suddenly leap on him from behind and start reducing him to bite-size chunks.
Frankly, it’s also an incredible spell for driving your DM totally nuts. It’s always satisfying when you skip past her carefully prepared traps and ambushes by simply opening a portal six inches away from the enchanted gemstone and getting out of there before the carefully-weighted pedestal has finished its dramatic trigger sequence.
If you get really creative and start really thinking with portals you can do some even crazier things with them. Stick one of the gates 500 ft. directly up in the air and team up with a grapple-specialist strong enough to toss people through the other entrance and you can turn it into a death trap.
Fighting at sea? Open it as far underwater as you can and clear the decks with a waterjet 10ft. across and twice as powerful as a riot cannon. Stick one end below a boulder and rain rocky death down upon your foes at more than 100 miles per hour.
The power of high-level spells and the scarcity of spell slots means that it can at times be hard to justify an Arcane Gate when there are more conventional options at hand, but few of them are as fun. And almost none of them make you feel as smug as you do when you pull off a particularly inventive use for it.
Not the most powerful spell for its level and certainly dependant on finding the right circumstance, but when you do the entire party is going to be grinning like idiots. Just hope your DM is too.
Casting time: 1 Action
Range: 500 feet
Components: V, S
Duration: Concentration, up to 10 minutes
You create linked teleportation portals that remain open for the duration.
Choose two points on the ground that you can see, one point within 10 feet of you and one point within 500 feet of you. A circular portal, 10 feet in diameter, opens over each point. If the portal would open in the space occupied by a creature, the spell fails, and the casting is lost.
The portals are two-dimensional glowing rings filled with mist, hovering inches from the ground and perpendicular to it at the points you choose. A ring is visible only from one side (your choice), which is the side that functions as a portal.
Any creature or object entering the portal exits from the other portal as if the two were adjacent to each other; passing through a portal from the nonportal side has no effect. The mist that fills each portal is opaque and blocks vision through it. On your turn, you can rotate the rings as a bonus action so that the active side faces in a different direction.