The Dark Angel is a giant stone obelisk located at the far end of the Devils Garden Trail. While most people turn around at Double O Arch, there is a well-marked primitive trail taking you one step further into the fascinating terrain beyond. There is a giant section of large sandstone fins alongside the tower that apparently hold dozens of Native American petroglyphs, but these are left off of the maps for preservation purposes. There are not many long hikes in Arches, so adding the Dark Angel to the Devils Garden is a must to keep it from going by too quickly. You also get some cool new perspectives of Double O Arch after passing by. The trail is mostly flat, so it doesn't add much more in terms of effort needed.
This hike has probably the greatest concentration of arches in the world including the largest spanning and some of the most approachable. Make sure to add the small side trips to Navajo and Partition Arches to further add to the length of this hike. Even with these side trips the total distance is still probably under 7 miles and doesn't have more than a few hundred feet of total elevation gain.
There are many cairns along the way to keep you on the trail. Keep an eye out for the many small lizards, deer and rabbits running around. Make sure to stay off the crytobiotic soil while enjoying the desert. Be careful for sun exposure and dehydration while in the desert.