We are building an image slicing integral field unit (IFU) to integrate into the IMACS wide-field imaging spectrograph on the Magellan Baade Telescope at Las Campanas Observatory. This instrument will be called the Reformatting Optically-Sensitive IMACS Enhancing IFU, or ROSIE IFU. The 50"×54" field of view will be pre-sliced into four 13"×54" subfields, and then each subfield will be divided into 21 0.6"×54" slices by an image slicer. ROSIE IFU will provide visible wavelength coverage of 1800 Angstroms at a spectral resolution R=2000. We are in the midst of mounting optics writing software. The ROSIE IFU will enable the efficient mapping of extended objects such as nebulae, galaxies, or outflows, making it a powerful addition to IMACS.