Inclusion Criteria:

        All participants:

          -  Age ≥18 years;

          -  Participants with histologically confirmed, locally advanced, unresectable, or
             metastatic solid tumors that progressed while on or after PD-1/PD-L1 containing
             therapy;

          -  Measurable disease;

          -  Eastern Cooperative Oncology Group (ECOG) Performance Status ≤1;

          -  Life expectancy estimated to be at least 3 months;

          -  Highly effective contraception;

          -  Willing and able to provide written informed consent.

        Expansion cohort only:

          -  Histologically and/or cytologically confirmed recurrent/metastatic (R/M) SCCHN that is
             not amenable to curative therapy by surgery or radiation;

          -  Only 1 prior anti-PD-1 or anti-PD-L1 therapy and documented PD-L1 scoring ≥1% by
             combined positive score or tumor proportion score as part of their treatment;

          -  An anti-PD-1 or anti-PD-L1 treatment regimen must be the last prior therapy before
             study enrollment, following no more than 2 prior systemic regimens for R/M SCCHN;

          -  Acquired resistance to an anti-PD-1- or anti-PD-L1-containing therapy;

          -  The participant agrees to undergo a pre-treatment and on-treatment core or excisional
             biopsy and the biopsy procedure is not judged to be high risk by the Investigator.

        Exclusion Criteria:

        All participants:

          -  Participant is deemed at high risk of fatal outcome in case of COVID-19;

          -  Participants with a history of COVID-19 and have not provided a negative test for SARS
             CoV-2 infection within 28 days of the planned first dose date with FS118;

          -  Prior therapy: Received systemic anti-cancer therapy within 28 days or 5 half-lives,
             of the first dose of study drug, or prior treatment with a LAG-3 inhibitor;

          -  Participants with active or documented history of autoimmune disease;

          -  History of uncontrolled intercurrent illness;

          -  Known infections;

          -  Uncontrolled CNS metastases, primary CNS tumors, or solid tumors with CNS metastases
             as only measurable disease;

          -  Prior history of or active interstitial lung disease or pneumonitis, encephalitis,
             seizures, severe immune related adverse events with prior PD-1/PD-L1 containing
             treatments;

          -  Significant cardiac abnormalities;

          -  Significant laboratory abnormalities;

          -  Intolerance to the investigational product or its excipients, or any condition that
             would significantly impair and/or prohibit the participants's participation in the
             study, as per the Investigator's judgment.

        Expansion cohort only:

          -  Participant has nasopharynx or thyroid primary tumor site;

          -  History of severe immune-related toxicity during the prior treatment with checkpoint
             inhibitors.