Rethnar the wizard was uneasy. He and his companions, the warrior Dreden, and the bard Yornoth, had discovered a ruined city. And so, curious of the possibility of riches within the ruined gates, Yornoth and he had boosted Dreden up the wall, and Dreden had pulled them up with the rope. Dreden had let them down the other side with the rope, and had jumped over the wall himself.
The group had wandered the city, and fought off some of the new inhabitants (a couple of trolls and an ogre had decided that city life was for them) as well as old inhabitants (a crazed gang of apocalypse survivalists had decided that life outside a city was for noobs). And now they were in the city park.
Trees dotted the grass. Bushes and low-lying plants had taken over the edges of the area. And in the center was a paradise.
It had perfectly placed trees. And the flowers... oh the flowers. They were scattered over the area, adding dots of color among the green grass, and they looked gorgeous, as if pulled straight out of a story-book.
Naturally, being learned as he was, this made Rethnar uneasy. He had learned that paradises were usually places where you got killed.
But Dreden and Yornoth weren't uneasy. They had taken one look at the scene and had run through it, laughing like children. After Yornoth had played some music, Dreden had found it had annoyed him, and the two were roughhousing. Wait a minute... Rethnar thought.
The music had been sub-par for the bard's standards. And Dreden should have won the roughhousing contest by now.
But there they were, prancing and rolling through the grasses and flowers like children.
Rethnar looked at the dark alleys around them. Even darker shapes were among them. He tried to rouse his allies, but they looked up at him with big, bulging, eyes, eyes that were innocent and had not seen the torments of battle. Eyes that were childish.
"Five more minutes?" whined Yornoth.
"No!" cried Rethnar gazing around wildly for the source of this magic. "We have no more time for games."
Okay, thought Rethnar. He recognized (barely) the trees. They were good.
Dreden and Yornoth had run away, forgetting Rethnar's command. And Yornoth had picked a flower and placed in Dreden's ear. And then Rethnar thought to himself, as the recognizable stoop of an ogre, and the rocky shell of a troll emerged from the surrounding shadows, the flowers...
The Flowers of Innocence are an interesting plant. They tend to thrive in ruins. In those ruins, they tend to be near other plants. If a river runs through, or there was once a city park, that is where they could be found.
It has been speculated by the wizarding community that the flowers are the epitome of the flower. The perfect flower. But, all things being in a balance, they need to be balanced out. Perfection, according to human standards, in, and thus the opposite of that perfection must also come in. Which, in part, explains the Flower's need for blood and suffering.
How best to gather the blood? The Flowers used to simply wilt and die in the ruins and the parks. Such places are a hotbed for sorrow, and the Flowers could, for a time, live off that. But then the evolved. Slowly, surely, they developed the capacity for Innocence. They would make the human passerby de-age in the mind. They would become more like children.
Oh, some people could resist the effects. It usually takes someone strong in the mind to resist. A strong will-power, a strong sense of bardic lore (alas, in the above example, Yornoth was more of the singing disposition and less of the story-telling), or another such path.
This leads to the downside for the Flowers. If they fail to capture you in their thrall, they make the subject uneasy. But the human mind can't easily cope with sudden uneasiness, and so they'll grasp at a reason for this uneasiness. This might because they are in a strange place, a dangerous place, or it might be because they have heard of these flowers before.
Once the Flower has a person in its power, it will release a pheromone that attracts monsters to the scene. Depending on the nearness of monsters, it might take anywhere from a couple of minutes to an hour for them to come. It also depends on other factors, like wind speed, climate, and other such things that effect both the willingness for the monsters to get up off their lazy butts, speed of the pheromon to travel, etc.
But the mosnters will eventually come, and the people will die.