  We report on five years of 3-5 micron photometry measurements obtained by
warm Spitzer to track the dust debris emission in the terrestrial zone of HD
166191 in combination with simultaneous optical data. We show that the debris
production in this young (~10 Myr) system increased significantly in early 2018
and reached a record high level (almost double by mid 2019) by the end of the
Spitzer mission (early 2020), suggesting intense collisional activity in its
terrestrial zone likely due to either initial assembling of terrestrial planets
through giant impacts or dynamical shake-up from unseen planet-mass objects or
recent planet migration. This intense activity is further highlighted by
detecting a star-size dust clump, passing in front of the star, in the midst of
its infrared brightening. We constrain the minimum size and mass of the clump
using multiwavelength transit profiles and conclude that the dust clump is most
likely created by a large impact involving objects of several hundred km in
size with an apparent period of 142 days (i.e., 0.62 au assuming a circular
orbit). The system's evolutionary state (right after the dispersal of its
gas-rich disk) makes it extremely valuable to learn about the process of
terrestrial planet formation and planetary architecture through future
observations.
