  We present a detailed study of a transient in the center of SDSS1115+0544
based on the extensive UV, optical, mid-IR light curves (LC) and spectra over
1200 days. The host galaxy is a quiescent early type galaxy at $z$ = 0.0899
with a blackhole mass of $2\times10^7M_\odot$. The transient underwent a 2.5
magnitude brightening over $\sim120$ days, reaching a peak $V$-band luminosity
(extinction corrected) of $-20.9$ magnitude, then fading 0.5 magnitude over 200
days, settling into a plateau of $>600$ days. Following the optical brightening
are the significant mid-IR flares at $3.4$ and $4.5\mu$m, with a peak time
delay of $\sim180$ days. The mid-IR LCs are explained as the echo of UV photons
by a dust medium with a radius of $5\times10^{17}$ cm, consistent with $\rm
E(B-V)$ of 0.58 inferred from the spectra. This event is very energetic with an
extinction corrected $L_{bol} \sim 4\times10^{44}$ erg s$^{-1}$. Optical
spectra over 400 days in the plateau phase revealed newly formed broad
H$\alpha, \beta$ emission with a FWHM of $\sim3750$ km s$^{-1}$ and narrow
coronal lines such as [Fe VII], [Ne V]. This flare also has a steeply rising UV
continuum, detected by multi-epoch $Swift$ data at $+700$ to $+900$ days post
optical peak. The broad Balmer lines and the UV continuum do not show
significant temporal variations. The slow evolving LCs over 1200 days, the
constant Balmer lines and UV continuum at late-times rule out TDE and SN IIn as
the physical model for this event. We propose that this event is a `turn-on'
AGN, transitioning from a quiescent state to a type 1 AGN with a sub-Eddington
accretion rate of $0.017M_\odot$/yr. This change occurred on a very short time
scale of $\sim 120- 200$ days. The discovery of such a rapid `turn-on' AGN
poses challenges to accretion disk theories and may indicate such event is not
extremely rare.
