A Study to Test the Efficacy, Safety, and Pharmacokinetics of Rozanolixizumab in Adult Study Participants With Leucine-Rich Glioma Inactivated 1 Autoimmune Encephalitis

Inclusion Criteria:

* Study participant must be ≥18 to ≤89 years of age
* Study participant must be seropositive for leucine-rich glioma inactivated 1 (LGI1) antibody
* Study participant must have ≥2 seizures/week during the Screening Period or have experienced such seizures that stopped following high dose corticosteroids (500 to 1000 milligram (mg) methylprednisolone (MP) equivalent/day):

  * Either faciobrachial dystonic seizures (FBDS) with or without other focal (partial) seizures including focal to bilateral tonic clonic
  * Or focal (partial) seizures including focal to bilateral tonic clonic and fulfil the following new-onset Autoimmune encephalitis (AIE) criteria
* Study participant has initiated or re-initiated corticosteroids at a dose of 500 to 1000 mg MP equivalent/day within 42 days prior to randomization. Participants re-initiating corticosteroids are eligible only if re-initiation is due to seizure rebound and within the timeframe outlined. If the study participant has initiated a steroid taper, the study participant cannot receive an oral steroid dose lower than 40mg/day when randomized
* Study participant with onset of disease symptom between 0 to 12 months prior to Screening, per investigator's assessment.
* Study participant weighs at least 35 kg at Screening
* A female participant is eligible to participate if she is not pregnant, not breastfeeding, and at least one of the following conditions applies:

  i) Not a woman of childbearing potential (WOCBP) OR ii) A WOCBP who agrees to follow the contraceptive guidance during the treatment period and for at least 90 days after the final dose of study treatment

Exclusion Criteria:

* Study participant has a known hypersensitivity to any components of the study medication or any other anti-neonatal Fc receptor (FcRn) medications.
* Study participant has a confirmed prior diagnosis of epilepsy or new onset seizures that are unrelated to LGI1 autoimmune encephalitis (AIE) or has any known or suspected medical cause for the onset of seizures other than possible AIE
* Study participant has a known active neoplastic disease or history of neoplastic disease within 5 years of study entry
* Study participant has renal impairment, defined as glomerular filtration rate (GFR) \<30mL/min/1.73m2 at the Screening Visit
* Study participant has a clinically important active infection (including unresolved or not adequately treated infection) as assessed by investigator, including participants with a serious infection within 6 weeks prior to the first dose of investigational medicinal product (IMP)
* Study participant has a history of chronic ongoing infections
* Study participant has current unstable liver or biliary disease, per investigator assessment, defined by the presence of ascites, encephalopathy, coagulopathy, hypoalbuminemia, esophageal or gastric varices, persistent jaundice, or cirrhosis
* Study participant has positive tuberculosis (TB) test at the Screening Visit
* Study participant has a history of solid organ transplant or hematopoietic stem cell transplant
* Study participant has undergone a splenectomy
* Study participant has a current or medical history of primary immune deficiency
* Study participant has received a live vaccination within 4 weeks prior to the Baseline Visit; or intends to have a live vaccination during the course of the study or within 8 weeks following the final dose of investigational medicinal product (IMP)
* Study participant has previously received rozanolixizumab drug product
* Alanine transaminase (ALT), aspartate aminotransferase (AST), or alkaline phosphatase (ALP) are \>3x upper limit of normal (ULN)
* Study participant has a total IgG level ≤5.5 g/L at the Screening Visit
* Study participant has absolute neutrophil count \<1500 cells/mm\^3 at the Screening Visit