Well. That didn't go so well. In fact, it could be said to have gone distinctly badly. Though at least she knows what sorts of capabilities it has now, to some extent.
Pointing at the treant, Spreagadh begins to chant softly, clearly casting a spell of some sort, and one that takes more preparation than the simple gestures they've used up to now.
"North wind, north wind, old and chill, freeze this one who'd do me ill.
East wind, east wind, sister young. Shield me from the trap I sprung.
South wind, south wind, hot and strong, burn the one who did me wrong.
West wind, west wind, lover now, take all that I can allow.
Fire, fire, fire of the sun. Help me now to kill this one.
Water, water, of the sea, from my foe I ask you flee.
Earth upon which he now stands, cast from you its bloody hands.
Air on you did I first call; now break his limbs and let them fall."
As she starts to speak, a chill wind rises, blowing around the grove; it should at least partially freeze the treant's sap, presumably slowing it, and possibly causing cracks or small explosions as it expands.
With the next line, another wind springs up as well, a fierce gale centered on the tree the fae is in, though she herself appears to be in the eye of the storm.
The third line's effect isn't immediately apparent; certainly, no additional wind is forthcoming.
The fourth is somewhat more dramatic, a gentle breeze stirring the leaves around the Leannan, following which she slumps back in the tree, obviously drained of energy.
On the fifth, a javelin-like jet of fire spits at the treant from her finger; it's somewhere around the point where aluminum will burn, and should hit with a similar effect to a ballista bolt.
With the sixth, the unusually ambulatory plant should find itself desiccating rapidly, the water apparently being leached from him into the air and the ground.
The seventh should result in the treant's hands being quite roughly shoved from the ground, probably after being squeezed in fissures for a moment first.
And, when she speaks the final line, a howling storm begins to form, starting with a wind strong enough to tear branches from many of the trees around them, and uproot some of the smaller ones outright.