@JeremyECrawford Horizon Walker Is Planar Warrior ability need to use bonus each turn to deal Force damage or needed once when you select creature? If need a bonus action every turn, then it negates Dual Wielders fighting style. Intended or oversight?
— Carmine (@nirvanstryder) November 28, 2017
The Planar Warrior feature intentionally requires a bonus action each time you use it. #DnD https://t.co/etrdUn96lp
— Jeremy Crawford (@JeremyECrawford) November 28, 2017
The text pretty clearly indicates that each time you use the ability as a bonus action, the effect lasts only for that turn. Thus, you need to use a bonus action every turn you want to use this ability.
It effectively is a replacement and slight upgrade for Two Weapon Fighting.
You get an extra 1d8 damage, which is equivalent to the best damage that you could deal with an Offhand Weapon, and it upgrades at 11th to 2d8 damage. Combine this extra damage with a Two-Handed weapon with a larger damage die, and you’re dealing more total damage with one weapon than with two.
This damage is also done automatically when you hit, which is a slight upgrade over having to make two separate attacks and hit with both of them.
In the event of a miss, it becomes a slight downgrade, since you don’t get a 2nd attack roll for partial damage. You also can’t attack two separate targets.
That said, you still have the option to do Two Weapon Fighting if you want instead of this ability, so if you want more chances to hit for at least partial damage, or if you want to attack multiple targets, you can still do that as needed. These issues also become less of a concern once you hit 5th and get Extra Attack.
An important thing to note is that Planar Warrior works with any weapon attack, including ranged weapons. Characters using such weapons get a flat upgrade, since they (mostly) can’t use Two-Weapon Fighting anyway.
There is also very strong synergy from taking the Archery fighting style instead of Two-Weapon Fighting, as you get a +2 to ranged attack rolls, meaning a larger number of your attacks hit the target, meaning you get the extra damage from Planar Warrior more often.
One final benefit is changing the attack to force damage, which gives you the ability to overcome common monster resistances and immunities to bludgeoning, slashing, or piercing damage from non-magical weapons. Such resistances are particularly common in Extraplanar enemies, which are what the Gloom Stalker specializes in fighting.