The Arcane Trickster and Eldritch Knight get 3rd-level spells when they reach 13th level. By that time, other classes are casting 7th-level spells like Plane Shift and Teleport, but Phantom Steed is one of the 3rd-level spells that are still useful at high levels of play.
What does it do?
This spell allows you to summon a spirit horse with a speed of 100, which is faster than nearly any other mount in the game. It can be cast as a ritual, requires no material components, and lasts for 1 hour.
Cast this spell in advance of a battle to give yourself or the front-line combatants in your party improved mobility on the battlefield. On a Phantom Steed, the Barbarian is as mobile as the Monk.
Cast Phantom Steed every hour on the hour to give yourself an inexhaustible mount that can travel upwards of 90 miles in a single 8-hour day.
There’s no limit on the number of Phantom Steeds you can have at any one time, other than that the casting time of the ritual is 11 minutes. So you could summon five of them in 55 minutes and give the whole party spirit horses for 5 minutes. That’s 50 rounds of combat.
Depending on whether your DM will allow you to cast a spell as a ritual while riding a Phantom Steed (there is nothing in the rules against it), you can use this spell as a reliable way to move the whole party at a speed of 100 miles per day. Cast Phantom Steed 4 times to give 4 members of the party spirit horses for 16 minutes. Then while you’re traveling on those horses at 13 miles per hour, start casting it again. Cast the spell continuously for 8 hours to generate a fresh steed every 11 minutes.
Compared with other spells
An obvious comparison is with the spells Find Steed (a 2nd-level Paladin spell) and Find Greater Steed (a 4th-level Paladin spell). A Phantom Steed is faster than any creature that can be summoned with either of those spells, and does not require a spell slot when cast as a ritual. Whereas the steeds summoned by the paladin are permanent, the Phantom Steed can be summoned again and again at only the cost of a few minutes of time.
Other 3rd-level spells that improve battlefield mobility are Fly and Haste, both of which require concentration and a spell slot. Water Walk is another 3rd-level spell that improves mobility by allowing you to walk on water.
Since the Phantom Steed is a creature, those other spells can be cast on it. What’s better than a spirit horse? A hasted spirit horse that can walk on water. A Phantom Steed with Water Walk cast on it is three times faster than the fastest ships.
Phantom Steed is an incredibly versatile utility spell that can drastically improve your mobility on and off the battlefield.