The Best Season Ever: Black False-Hellebore
This perennial is one of hardy horticulture's enduring unicorns: It's rarely available for sale, is famously poisonous to every creature but the favored polinators, blooms with thousands of sepulchrally-dark flowers, and bears large obsessively-tidy, pleated green foliage.
Did I mention that it's painfully slow to mature to flowering age? That it's fatally at risk if the soil is anything less than deep, rich, and moist? Unicorn, indeed.