The royal sunangel is 10 to 12 cm (3.9 to 4.7 in) long and weighs 3.4 to 4.5 g (0.12 to 0.16 oz). Its black bill is short and straight. Both subspecies are strongly sexually dichromatic. The adult males are unique among genus Heliangelus : Their bodies are entirely dark blue. The royal sunange's plumage is slightly shiny, with significant iridescence only on its forecrown (just above the bill). H. r. johnsoni is intensely iridescent all over, especially so on the crown, throat, and upper breast. The male royal sunangel's tail is dark metallic violet-purple and that of H. r.johnsoni is metallic indigo. Both males' tails are deeply forked. Adult females of the two subspecies are similar to those of the rest of genus Heliangelus and differ only slightly between them. They are dark green above and rich cinnamon with bronzy green discs below. A buffy breastband separates the throat and chest. Their tails are not as deeply forked as those of the males.

The royal sunangel is mostly found in the Cordillera del Cóndor of Peru's Cajamarca and Amazonas areas, and also on the east slope of the Andes in Peru's areas of San Martín and Loreto. It was first documented in extreme southeastern Ecuador's Zamora-Chinchipe Province in 2008.

The royal sunangel feeds on nectar and insects. Several flowering plants, shrubs, and small trees have been identified as nectar sources. Males are highly territorial at flowering bushes. Both sexes often feed by perching on a flower rather than hovering.

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