The timescale of a microlensing event scales as a square root of a lens mass. Therefore, long-lasting events are important candidates for massive lenses, including black holes. Here we present the analysis of the Gaia18cbf microlensing event reported by the Gaia Science Alerts system. It has exhibited a long timescale and features characteristic to the annual microlensing parallax effect. We deduce the parameters of the lens based on the derived best fitting model. We used photometric data collected by the Gaia satellite as well as the follow-up data gathered by the ground-based observatories. We investigate the range of microlensing models and use them to derive the most probable mass and distance to the lens using a Galactic model as a prior. Using known mass-brightness relation we determined how likely it is that the lens is a main sequence star. This event is one of the longest ever detected, with the Einstein timescale of t_E_=491.41^+128.31^_-84.94_ days for the best solution and t_E_=453.74^+178.69^_-105.74_ days for the second-best. Assuming Galaxy priors, this translates to the most probable lens mass of M_L_=2.65^+509^_-1.48_M_{sun}_ and M_L_=1.71^+3.78^_-1.06_M_{sun}_, respectively. The limits on the blended light suggest that this event was most likely not caused by a main sequence star, but rather by a dark remnant of stellar evolution.