The mysterious race known to the Druids as "Travellers" come from some alternate PMP where psionic powers are much more commonly developed by that plane's "humans" than in the normal AD&D world. The actual "Travellers" are a group of these humans who have developed the psionic science of "probability travel" and combined it with a desire to explore the multiverse. It is surmised that most of the race stay at home and thus do not become Travellers, and there is no over-all name for the race, who's homeworld is unknown, at least on any world where magic works.
All Travellers have Int, Wis, and Cha of at least 13 (roll 4d6, discard lowest and move any score under 13 to 13), with one of the three being 16 (pick at random or choose); their other ability scores are in the normal 3-18 (just roll 3d6) range. Psionic strength is calculated using 1d20x5 plus the normal bonuses given in PHB.
Travellers select their attack and defence modes in order (ie, a Traveller with 2 attack modes must take A and B; one with 4 defence modes must take F-I).
Disciplines are rolled on the PHB table using 1-60 (d3 + d20) plus 40, and probability travel must be selected as a major discipline.
The level of a Traveller when encountered will be 1d6+5 (or enough to have obtained all their rolled disciplines, whichever is higher) and they generally function as magic users for purposes of which psionic disciplines and magic items they may use, but they have 8-sided hit dice and may receive up to 2hp per level CON bonus. They may use any weapon but receive a -4 penalty for any melee weapon (they use missile weapons normally).
Their homeworld is technological rather than magical to a degree that they suffer/benefit from the same relationship to magic as dwarves do in respect of magical saving throws and difficulties using certain items (they have no special bonuses as regards poison, however).
They regenerate 1hp per turn so long as they are alive.
Those encountered will be Chaotic but may be of any morality from Good to Evil, and 90% are male (due to socio-political issues on their homeworld, it is assumed, although perhaps that is just the sex-ratio of their race). They tend to travel alone, but the more powerful practitioners may use some of their psionic ability to bring non-Traveller companions along on their adventures across the worlds.
Although able to travel to the Outer Planes, the Travellers are known to avoid such and it is thought that their technological background somehow makes such locations more dangerous for them in some way. They may be encountered on the Inner Planes with more regularity but even then only if the Traveller's disciplines will are those that make the environment more survivable for them.
Travellers have various reasons for their travelling and some are rivals while others friends or lovers of sorts, but no example is known of two Travellers regularly travelling together. They are uniformly tight-lipped about life on their homeworld and do have a tendency to treat other intelligent beings with a touch of condescension. If they join a party, it will always be as leader (this may not be stated up-front, but it will always be assumed by the Traveller).
Each Traveller will have fields of interest as per a sage but they travel light and may or may not have facilities equal to a library with them when determining chances to know specific information.
Due to the requirement that the Traveller is the leader, any characters who agree to join them become NPC henchmen under the Traveller's direction. This does not preclude "short-term engagements"; basically single adventures.
A Traveller is posing as a sage in a city and has been buying items off adventuring parties which correspond to his/her field of interest. Players being what they are, it will only be a matter of time before they attempt to rob the Traveller. The Traveller's companions may also turn up as hired help for parties investigating particular areas/legends relevant to the Traveller.
A Traveller sans companions materializes by a party who are lost or otherwise desperate and offers to transport them to safety if they first help him/her investigate some nearby site. Depending on the Traveller's alignment, they may not honour this but in any case a successful outcome may result in an offer to take either PCs or henchmen on as companions.
The party come across a village decked out in Nazi insignia. A Traveller has studied Hitler and other fascists and decided that the system is a good one but only where there is no threat from other technologically advanced nations. Using their psionics, they have dominated (perhaps literally using that power) this area as a test bed. The local blacksmith has made a crude tank powered by steam under direction of the Traveller. Currently it fires nothing more effective than steam (3" long cone, 2" wide at terminal, 2d6 damage, save against breath weapon for half) but the Traveller is considering taking on a magic-user companion to beef it up a bit.
An injured and slightly delirious Traveller is encountered who is being pursued by "silver suits"; s/he is keen to get in touch with "Giji and his sons". What these things may be is up to the DM.
An evil Traveller has taken up a position of power in a neighbouring area (probably as the power behind a throne) and is spreading their influence over the surrounding territory. The PCs are contacted by a noble who has lost his lands and wishes to hire a group to investigate and overthrow this new power. The noble is eventually revealed to be another Traveller, a long-standing foe of the evil one. If the evil one flees this world, the good one will follow, possibly after offering companionship if the party has been helpful.
A Traveller contacts the party, seeking "those who can deal with the exceptional". The barest of plans will be outlined but the target is apparently some artefact of great power guarded by many terrible monsters whom the Traveller describes as "nothing you have ever encountered before". The Traveller is not worried about these monsters and in fact seems eager to get started, excited even. The size of the party is limited by the Traveller's probability travel power as the artefact in question is on The Starship Warden.
The party is openly contacted by a Traveller who wishes to be shown the "unique flora and fauna of this world" and is willing to pay in both gold and a smattering of "magic items" which may, in fact, be technological items such as video phones, laser pistols, super-insulated tents and other similar equipment which is useful for adventuring.
And so forth... Eventually the Gay Deceiver will make an appearance and then all bets are off!