  Thanks to the advent of large-scale optical surveys, a diverse set of flares
from the nuclear regions of galaxies has recently been discovered. These
include the disruption of stars by supermassive black holes at the centers of
galaxies - nuclear transients known as tidal disruption events (TDEs). Active
galactic nuclei (AGN) can show extreme changes in the brightness and emission
line intensities, often referred to as changing-look AGN (CLAGN). Given the
physical and observational similarities, the interpretation and distinction of
nuclear transients as CLAGN or TDEs remains difficult. One of the obstacles of
making progress in the field is the lack of well-sampled data of long-lived
nuclear outbursts in AGN. Here, we study PS16dtm, a nuclear transient in a
Narrow Line Seyfert 1 (NLSy1) galaxy, which has been proposed to be a TDE
candidate. Our aim is to study the spectroscopic and photometric properties of
PS16dtm, in order to better understand the outbursts originating in NLSy1
galaxies. Our extensive multiwavelength follow-up that spans around 2000 days
includes photometry and spectroscopy in the UV/optical, as well as mid-infrared
(MIR) and X-ray observations. Furthermore, we improved an existing
semiempirical model in order to reproduce the spectra and study the evolution
of the spectral lines. The UV/optical light curve shows a double peak at
$\sim50$ and $\sim100$ days after the first detection, and it declines and
flattens afterward, reaching preoutburst levels after 2000 days of monitoring.
The MIR light curve rises almost simultaneously with the optical, but unlike
the UV/optical which is approaching the preoutburst levels in the last epochs
of our observations, the MIR emission is still rising at the time of writing.
The optical spectra show broad Balmer features and the strongest broad Fe II
emission ever detected in a nuclear transient. [abridged]
