  This paper presents a new optical imaging survey of four deep drilling fields
(DDFs), two Galactic and two extragalactic, with the Dark Energy Camera (DECam)
on the 4 meter Blanco telescope at the Cerro Tololo Inter-American Observatory
(CTIO). During the first year of observations in 2021, $>$4000 images covering
21 square degrees (7 DECam pointings), with $\sim$40 epochs (nights) per field
and 5 to 6 images per night per filter in $g$, $r$, $i$, and/or $z$, have
become publicly available (the proprietary period for this program is waived).
We describe the real-time difference-image pipeline and how alerts are
distributed to brokers via the same distribution system as the Zwicky Transient
Facility (ZTF). In this paper, we focus on the two extragalactic deep fields
(COSMOS and ELAIS-S1), characterizing the detected sources and demonstrating
that the survey design is effective for probing the discovery space of faint
and fast variable and transient sources. We describe and make publicly
available 4413 calibrated light curves based on difference-image detection
photometry of transients and variables in the extragalactic fields. We also
present preliminary scientific analysis regarding Solar System small bodies,
stellar flares and variables, Galactic anomaly detection, fast-rising
transients and variables, supernovae, and active galactic nuclei.
