You have more than one identity. The one you wear most of the time makes you appear to be a member of a guild other than House Dimir. You have documentation, established acquaintances, and disguises that allow you to assume that persona and fit into the secondary guild.
Whenever you choose, you can drop this identity and blend into the guildless masses of the city.
Consider why you're embedded in the secondary guild. Create a story with your DM, inspired by rolling on the following table or choosing a reason that suits you.
1 - My parents belong to this guild, and I let them think I'm following in their footsteps.
2 - I've been assigned to track this guild's activities.
3 - I've been assigned to get close to an individual in this guild and learn their secrets.
4 - I've been assigned to recruit a new Dimir spy from the ranks of this guild.
5 - I was a member of this guild before the Dimir recruited me.
6 - I don't like what this guild stands for and want to destroy it from within.
7 - I secretly wish I could leave the Dimir and join this guild, but there is no escaping the Dimir.
8 - I chose this guild at random or on a lark.
For you, the spells on the Dimir Guild Spells table are added to the spell list of your spellcasting class. (If you are a multiclass character with multiple spell lists, these spells are added to all of them.)
Cantrip - Encode thoughts, mage hand
1st Level - Disguise self, sleep
2nd Level - Detect thoughts, pass without trace
3rd Level - Gaseous form, meld into stone, nondetection
4th Level - Arcane eye, freedom of movement
5th Level - Modify memory
Your magic is meant to be subtle and undetectable, but it might pull shadows or clouds of mist around you as you cast your spells. Using the encode thoughts cantrip described below, you can turn a creature's thoughts (including your own) into a thought strand that others can potentially read, share, or steal. These thought strands are treated as valuable currency among the Dimir.
Skilled at infiltration, disguise, and deception, members of House Dimir appear inscrutable. Your true personality and ideals might never manifest, or they might mark you as a quirky member of your secondary guild.
1 - I'm good at hiding my true thoughts and feelings.
2 - When I'm in doubt about revealing something, I assume it's a secret, and I don't share it.
3 - I like to sound mysterious, because wisdom hidden grows deeper with time.
4 - I have no patience with people who get in my way.
5 - I love hearing about other people's nightmares.
6 - Combat is meant to be quick, clean, and one-sided.
7 - I like to stick to the shadows.
8 - I never show my anger. I just plot my revenge.
1 - Guild. My true guild is all that really matters. (Any)
2 - Control. I like pulling the strings. (Lawful)
3 - Secrets. I collect secrets and never reveal them. (Any)
4 - Knowledge. I want to know as much as I can about this city and how it works. (Any)
5 - Independence. I value the freedom to pursue my own goals without interference. (Chaotic)
6 - Nihilism. I don't believe in anything, and anyone who does is a fool. (Neutral)
1 - I discovered a secret I can't let anyone else uncover—including my guild superiors.
2 - I formed a close friendship or romance with someone in the guild I'm infiltrating.
3 - The Dimir agent who recruited me was unmasked and killed. My revenge on the killers will be thorough and painful.
4 - I spend as much time as I can in the Ismeri Library because I'm certain an information hub operates behind its facade. I want its secrets!
5 - I'm utterly loyal to my superior in the guild, more than to the guild or its guildmaster.
6 - Someone has discovered my true identity.
1 - I like secrets so much that I'm reluctant to share details of a plan even with those who need to know.
2 - I would let my friends die rather than reveal my true identity.
3 - I have trouble trusting anyone but myself.
4 - I have a particular vice that puts all my secrets at risk if I'm not careful.
5 - I'm pretty sure I've done something horrible that I can't remember because of the guild's mind magic.
6 - I put too much trust in the people who give me orders.
As an agent of House Dimir working undercover, you have limited contacts within your guild. Your relationships within your secondary guild, in the guise of your false identity, are usually more extensive.
Roll once on the Dimir Contacts table, giving you an ally who serves as your contact in Dimir. Then roll twice on the table for your secondary guild. The first roll gives you an ally there, and the second roll gives you a rival.
1 - I know a Dimir courier who relays messages to me from someone higher up the chain of command.
2 - I get orders from a shapeshifter I recognize only through a series of code phrases we exchange.
3 - An ostentatiously wealthy vampire is my secret guild superior, summoning me to a luxurious estate by means of coded messages.
4 - I have never met my guild contact, but I receive telepathic messages, usually in my dreams.
5 - I've never met my guild contact, but I get coded messages from a pattern of street lights and graffiti.
6 - I didn't discover the identity of my guild contact until after we had begun a romantic relationship.
7 - My superior maintains an elaborate identity as a young street urchin... unless it's all a lie, and I'm being sent on ridiculous missions by a twisted child.
8 - My sibling and I both get telepathic orders from a mysterious contact, and I'm starting to question the authenticity of my sibling's orders.
As a Dimir adventurer, you are a member of the guild's network of spies, thieves, assassins, and mind mages that lurks behind the facade of the public guild. On the surface, House Dimir presents the appearance of a network of couriers, investigators, media reporters, and archivists, dealing in information and spreading news. But you and your peers trade in secrets. You use secret symbols, runes, and signals to surreptitiously communicate with other Dimir agents, often in plain sight.
Like any good spy, you have multiple identities: your true face as an agent of House Dimir; a guildless identity; and a role as a member of another guild. Within that secondary guild, you might already be on a mission for House Dimir, assigned to spy on the guild, collect information about a person, or recruit another spy from the ranks of the guild. Or that guild could be a launching point for your real mission. Perhaps, for example, you were ordered to infiltrate the Azorius in hopes of gaining access to a notorious inmate in an Azorius prison.