Background The public health impact of Chikungunya virus (CHIKV) is often underestimated. Usually considered a mild condition of short duration, recent outbreaks have reported greater incidence of severe illness, fatality, and longer-term disability. In 2018, Eastern Sudan experienced the largest epidemic of CHIKV in Africa to date, affecting an estimated 487,600 people. Known locally as Kankasha, this study examines the clinical characteristics, risk factors, and phylogenetics of the CHIKV epidemic in Kassala City. Methodology and Principal Findings A prospective cohort of 142 cases (102 adults, 40 children) were enrolled at Kassala Teaching Hospital in October 2018. Clinical information, socio-demographic data and sera samples were analysed to confirm diagnosis, characterise illness, and identify the viral strain. CHIKV infection was confirmed by real-time reverse transcription-PCR in 84·5 percent of participants. Nine had concurrent CHIKV and Dengue virus (DENV) infection and 28·8 percent had a positive Rapid Diagnostic Test for malaria. Five percent had haemorrhagic symptoms including two children with life-threatening haemorrhage. One CHIKV-positive participant died with acute renal injury. Ninety to 120 days post-illness, 63 percent of those followed-up were still experiencing arthralgia in one or more joints, and 11 percent remained moderately disabled using Rapid3 assessment. Phylogenetic analysis showed all CHIKV infections belonged to a single clade within the Indian Ocean Lineage (IOL) of the East-Central-South African (ECSA) genotype. History of contact with an infected person was the only socio-demographic factor associated with infection (p 0·01), suggesting that vector transmission in households is important. Conclusions and Significance The epidemic is estimated to have affected approximately 50 percent of Kassala City's population. Substantial vulnerability to CHIKV remains here and elsewhere in Sudan due to widespread Aedes aegypti presence and mosquito-fostering household water storage methods. This study highlights the importance of increasing awareness of the severity and socio-economic impact of CHIKV outbreaks and the need for urgent actions to reduce transmission risk in households.