I'm creating factions out of sub-classes from each class in Xanathar's Guide to Everything, and today we come to the ranger. The one sub-class that leapt out to me was the Horizon Walkers, rangers who walk the planes and protect the gateways between them Perfect!
They call themselves "Those Who Walk Between." Heavily cloaked and nearly always wielding staves or other pike weapons, these mysterious entities will stay overnight at a tavern in one country, then appear at a farmhouse hundreds of miles away the next day. Most are quiet types, but some open up to those who bribe them with a good drink or three, and tell stories of wild adventures on other worlds and realms of existence.
Those Who Walk keep tabs on the many portals that dot the multiverse, and follow up on reports of new ones. They both discreetly check on official teleportation circles to ensure they're stable, and ensure that out-of-the-way portals haven't been claimed by nefarious forces or destroyed.
How do they have the time to do this? An even more mysterious group of ancient wizards who call themselves the Onlookers fund Those Who Walk Between. Each member of the latter group occasionally meets with one or another of the Onlookers to receive payment and leads on unmapped portals.
As a friendly faction, a member of Those Who Walk Between might seek out your party and become its patron, handing out portal-related quests (such as finding an abandoned portal, activating a portal recently found, or wiping out a group of enemies who've claimed a portal site for their own).
Those Who Walk can also serve as temporary allies in a particular story arc, appearing in a time of crisis to offer aid to a particular faction.
As a foe faction, Those Who Walk Between see portals as dangerous technology that should be kept away from mortals. They lay traps, set up guardians, and put invisibility spells around any portals they protect. If the PCs find and use an out-of-the-way portal, they may find themselves attacked by one of Those Who Walk, or by assassins sent by them.
- (Friend) One evening while preparing dinner, a robed figure appears in the middle of their camp. The figure reveals itself to be an elven woman, and introduces herself as "One of those who walk between." She explains that she watches over a portal that lies at the top of a ruined tower nearby, and she was checking up on it a few days ago to find a tribe of orcs had taken up residence. She offers a reasonable monetary reward if the PCs will wipe out the orcs, as well as occasional use of the portal (which can lead to a place the PCs particularly need to go to right now).
- (Foe) A group of monks has found an old portal to the Astral Plane and begun to build a monastery around it. However, they work has gone slowly as one disaster after another has befallen them, from materials stolen overnight to attacks from monster uncommon in that area. Now they're finding some of their work undone each morning, and they suspect sabotage. They ask the PCs to investigate, and they find that a less aggressive member of Those Who Walk Between has been trying to discourage the monks enough that they'll abandon the project. If the PCs escalate, their foe can call in some wild backup from across the multiverse, but will not fight to the death; instead she will escape and work to destroy the portal.