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We systematically analyze X-ray variability of active galactic nuclei (AGNs)in the 7~Ms \textit{Chandra} Deep Field-South survey. On the longest timescale($\approx~17$ years), we find only weak (if any) dependence of X-rayvariability amplitudes on energy bands or obscuration. We use four differentpower spectral density (PSD) models to fit the anti-correlation betweennormalized excess variance ($\sigma^2_{\rm nxv}$) and luminosity, and obtain abest-fit power law index $\beta=1.16^{+0.05}_{-0.05}$ for the low-frequencypart of AGN PSD. We also divide the whole light curves into 4 epochs in orderto inspect the dependence of $\sigma^2_{\rm nxv}$ on these timescales, findingan overall increasing trend. The analysis of these shorter light curves alsoinfers a $\beta$ of $\sim 1.3$ that is consistent with the above-derived$\beta$, which is larger than the frequently-assumed value of $\beta=1$. Wethen investigate the evolution of $\sigma^2_{\rm nxv}$. No definitiveconclusion is reached due to limited source statistics but, if present, theobserved trend goes in the direction of decreasing AGN variability at fixedluminosity toward large redshifts. We also search for transient events and find6 notable candidate events with our considered criteria. Two of them may be anew type of fast transient events, one of which is reported here for the firsttime. We therefore estimate a rate of fast outbursts $\langle\dot{N}\rangle =1.0^{+1.1}_{-0.7}\times 10^{-3}~\rm galaxy^{-1}~yr^{-1}$ and a tidal disruptionevent~(TDE) rate $\langle\dot{N}_{\rm TDE}\rangle=8.6^{+8.5}_{-4.9}\times10^{-5}~\rm galaxy^{-1}~yr^{-1}$ assuming the other four long outbursts to beTDEs.

Deepest view of AGN X-ray variability with the 7 Ms Chandra Deep Field-South survey