My understanding of Elana's lore.
Everyone already knows the story of the four daughters of darkness; they are all fragments of Manus, born from his death, taking the form of human women and allying themselves (or at least attempting to) with some king in the past.
However, it's noticeable how each one represents an aspect of humanity, and all have almost clear objectives.
Nashandra was the smallest of the fragments and sought precisely what she lacked, power; she represents ambition.
Nadalia searched for a king, but he had long since disappeared by the time she arrived, so she clung to what she had of him, his tower, his relics. Nadalia represents solitude.
Alsanna directly states that she represents her father's fears; when the fragments were separated, she felt she would disappear, but she found a comfortable place to stay beside the ivory king.
And we have Elana representing Wrath... and that's all :D.
Okay, she sings to the dragon Sinh, amassing souls for the coming day of vengeance. But vengeance against whom? Against the Drakeblood Knights for attacking the dragon? Okay, that would make sense if the Knights weren't in such a deplorable state now; their leader died attacking Sinh, and the rest must have died thanks to the poison cloud that engulfed the town. The few survivors have all turned Hollow.
Elana is supposed to represent Wrath, so it doesn't make much sense to me that she only demonstrates this after all the chaos has happened, while her sisters demonstrate their aspects all the time.
Wrath and common anger are not the same thing. Wrath is linked to something more divine, just, or that there is a reason for it to happen.
What I'm starting to believe is that Elana always wanted this to happen in the first place, and this is her vengeance.
The descriptions say that when the Drakeblood Knights' leader's spear pierced the dragon's chest, the poison that had accumulated inside gushed out, poisoning all of Shulva but purifying the dragon. If the dragon was purified, that means the poison was never its own, so how did that poison end up on it?
The Sunken King created this city to worship the Dragon, and all methods of defense, including the soldiers, were created to stop the Drakeblood Knights, as they believed that dragon's blood would allow them to transcend existence.
The way this is handled leads us to understand that the attack described wasn't the only one. Attacking and killing dragons isn't new in Dark Souls; if there's a dragon, there will always be someone wanting to kill it.
I think Elana already knew this, and that's exactly why she's here. It's never said where she came from, when she appeared, or what she wants. She just popped up in Shulva's story out of nowhere.
I believe she came to these lands precisely because she already knew of Shulva's conflict against the Drakeblood Knights. She gained the Sunken King's trust and became a priestess of the sanctuary, singing to the sleeping dragon, but secretly accumulating poison within its heart.
If the dragon were attacked, the poison would spray directly onto the attackers, completely decimating them; and considering the pattern of events this would happen sooner or later.
Since Elana is the "augur" of Wrath, daughter of the abyss and the "Squalid Queen" this is probably exactly what she wants; Shulva's fate is merely a consequence, irrelevant to her plans as long as she delivers what she intends to
The Sunken King ends up being just a victim of a stray bullet in this story, or a means to an end for Elana, since he only made things easier for her, and I doubt she really cared about him.