Nevados de Chillán volcanic complex (NChVC) is one of the most active volcanoes in Chile having 4 eruptive episodes within the previous century. All of this activity has been concentrated within Las Termas sub-complex, the last eruption before the current reactivation occurred in 2008 characterized by a lava flow toward the east side of the edifice. The first surficial manifestation was an explosive event occurred on January 8, 2016, NChVC has been in eruption since then and continuing today. Seismicity has shown a variety of waveforms and types: Long Period (LP), Tornillo (TO), Very Long Period (VLP), spasmodic tremor, harmonic tremor and Volcano Tectonic (VT) earthquakes. Reactivation was marked by an increase in LP seismicity accompanied by a more regular VT activity. Based on seismicity, morphological changes and some petrological features it has determined five phases of activity, from hydrothermal dominance up to different magmatic episodes. It includes a lava dome extrusion, alternate growing and destruction of it, effusion of four lava flows. It is remarkable the low flow rates observed on extrusion and effusion of lava bodies (0.008 m3/s-0.04 m3/s), the constant vulcanian type eruptions of low and middle intensity, the occurrence of piroclastic flows mainly toward north and northeast to the crater and deformation process absent in the lava dome extrusion and very clear during lava flows effusion. This work makes a broad description of multiparametric data acquired by the monitoring network from 2012 to 2020, in order to illuminate the internal and external processes acting during the NChVC crisis and a hypothetical model of the magmatic plumbing system is proposed. This model states the presence of a magma plug in the shallow (0.5–1.1 km) conduit reactivated by at least two intrusive processes from 4 to 6 km depth magma chamber following the trend of a major fault structure, Cortaderas fault, which controls the distribution of eruptive centers of the complex. Our results could contribute also, to understand the behavior of the magmatic systems that produce dacitic lava domes with low volume and low growing rates.