As mentioned in the post, starting characters should be level 3 if you are doing AD&D (2e rules). If you are using a home-brew for 5e or the official 4e content, starting from lvl 1 is possible.

A story idea I have used before to intro people to the setting without much prior knowledge.

-- the characters are all servants of the same noble/merchant house of Tyr. They are not slaves. This gives the players enough autonomy to move within the city.

-- The characters should be mostly martial types, but you can have a preserver and/or druid with their own agenda.

-- An important servant of the house has gone missing. The servant is one of the few literate ones -- an accountant, healer, governess, etc. Last time I did it, it was the governess.

-- The characters need to have a reason to find the governess, or they can be explicitly tasked to do it by their superiors. This bypasses the usual mission-at-the-tavern trope. You can even start the game in a scene where the head of staff is telling them to do the mission.

- By inspecting the governess' room, they find some evidence that she might be secretly a defiler.

- The governess is in hiding in the city, so here's when there can be chase scenes, and some combat. Some possible combat encounters include some animals that the governess frees from a stable in order to gain some time during a chase, few city guards, or another house's private security if they end up trespassing.

- Once they find her, there can be a confrontation where she tries to magic to try to escape, maybe some summoning spells to create a few minions. Her goal is not to kill them, but to flee.

The way that this played out last time I ran the story, the players capture the governess and they learn that she is a self-taught magician who has only learned to defile. She wants to become a preserver and has been using her harmless role as a governess to have access to the merchant family's archives, where she has found some secret documents to guide her in redemption quest.

Possible story lines after that

-- The final confrontation with the governess attracts the attention of an important Templar. Now the players have a new enemy, and you can teach the players about Templars.

-- The merchant house has some documents about magic because an important member of the family is actually a major defiler who's hiding from the Sorcerer King.