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We present the instrument simulator xifusim developed for the X-ray Integral Field Unit X-IFU aboard the planned Athena mission. xifusim aims to be an accurate representation of the entire instrument, starting from a full simulation of the Transition-Edge Sensor (TES) array receiving impact photons unconstrained by the small signal limit. Its output current is then propagated through the entire readout chain, including multiplexing, amplification and the digital readout. The final output consists of triggered records, which can be post-processed to reconstruct the photon energies. The readout chain itself is separated into individual, modular blocks with several possible models for each, allowing the simulation of different readout schemes or models of varying physical accuracy at the expense of run time. New models are implemented as necessary to enable studies of the overall readout chain. Such studies are also facilitated by fine-grained control of the simulation output, including the internal state of intermediate simulation blocks. In addition to its modularity, xifusim also allows the manipulation of certain internal parameters during a run, enabling the simulation of readout chain characterization measurements, environmental drifts or various kinds of crosstalk.