See also: Seeming, a more powerful version of this spell.
Disguise Self is a level 1 illusion spell. It allows spellcasters to temporarily change their appearance. While active, gain access to the action Dispel Disguise to end the effect.
Given the spell's categorization as an illusion rather than a transmutation, and based on its original description and limitations in Dungeons & Dragons 5th Edition, one would expect this spell to behave primarily as illusion-based in Baldur's Gate 3, but this is not the case, as is evidenced further below.
However, in many other respects, this spell does cause the game to treat the disguised character as fundamentally that of their new presence, including in some race related distinctions. Known examples of this include:
Assuming the actual size of the disguised form; if the selected form is a race with a different creature size, it can be used to fit through some entrances with size requirements.
The racial descriptor at the top of the character sheet changes to the race the character is disguised into, and idle animations of that race are used.
Being able to use Speak with Dead on creatures the caster has killed before changing disguises.
Being able to repeat certain dialogue options with companions and NPCs, though this does not allow for changes in approval which already occurred.
For Clerics and Paladins of Selûne, being able to safely pray at the Altar to Shar and obtain her blessing, rather than her curse.
A separate attitude score is used, and permanently maintained, for each separate disguise used with merchants.
Race specific dialogue options in both companion and NPC interactions (e.g., Crèche Y'llek's Antechamber) often become available. In some cases, there are plot-specific dialogue options specifically related to disguising oneself as another race (e.g., pretending to be githyanki in an attempt to bypass their patrols). In other cases, the dialogue makes no distinction between disguising oneself as a race and actually being a member of it.
Characters can use this spell to escape each prison once. By changing appearance after being jailed – either by donning or shedding a disguise – the Fugitive condition no longer triggers guard recognition. However, characters who revert to their "criminal" form before the condition expires will blow their cover, and guards will see through any subsequent attempts to use this trick in the same prison.
Disguised companions and hirelings revert to their normal form upon leaving the active party.
Bugs
The player character (selected) and their disguised companions, standing together in camp.
If a companion or hireling is disguised while at camp, they will remain where they were last placed when another companion or player character is selected, rather than automatically moving back to their tent, until they are moved again or a long rest is taken. This can be useful when seeking to apply conditions or spells with a limited radius, such as Aid.
As of Patch 5, Hotfix 17, it is possible to give the caster racial equipment bonuses for longer than intended by using the Mask of the Shapeshifter, taking the mask off, and long resting without removing the disguise. On waking up, the disguise will be gone, but the racial bonuses on the equipment will still affect the character. This only works for racial equipment bonuses, not racial dialogue options.
If the player character is disguised as a githyanki when talking to Shadowheart for the first time on the beach at the start of Act One they are awarded an extra 30 xp.
While hirelings are disguised, they cannot be spoken with.
While disguised, if companions are slated to speak during cut-scenes, they cannot be seen or heard, though their lines can still be read on-screen.
