Just a little context to Vellian's bloody raids twenty-five years ago --
When she first took over the Underworld, she rallied the Underworlders and led them to the surface for quick, concentrated attacks against isolated Agrarian civilians. These raids focused mostly in the Rosaceae, Gardenia, and Convallaria regions. (See map in Gallery or click here) Vellian knew she didn't have the manpower to face the full force of the Agrarian army and leaned on guerilla-style warfare to assert herself as a power to be reckoned with.
She also wanted to be feared. Her troops could rise up from practically any cave/underground area as the Underworld is mostly connected under the southern half of the continent. So she could arrive suddenly, inflict devastating damage, killing many, then disappear.
But she is remembered most for her literal bloodlust. She would sometimes drink the blood of her victims, which is how Juliana died. This tactic was mostly done to incite fear, but her plan worked, and she became a legendary, mysterious monster in Agrarian lore and is still feared by most.
Also! Fun fact, you've seen Oliver's letter before.