Element of Honesty
    Location: The LIMINAL KEEP, under the Lake of New Canterlot.
    Power when Corrupted: False divination (tending to drive seers to despair)
    
    The element is kept with equipment to facilitate scrying in the dome of the
    Liminal Keep, Bluestar’s sanctum sanctorum. Access to the Liminal Keep
    requires exploration in New Canterlot, including possibly talking to NPCs,
    and figuring out how to access the keep (involving the hidden water level regulating 
dam. Bonus points for not draining the Lake.)

Bluestar himself found the Element in the periapocalyptic period and kept it in his
secret fortress. He knew of, and wrote about, its corruption. His journal, explaining
that the element is corrupted, can be found in the Liminal Keep, in his quarters.

After the Element of Honesty was lost by its bearer, it was found by sea ponies and brought to the archmage of the Isle of Yore, Bluestar the Brillant, for safekeeping. Bluestar recognized that the element had been corrupted, and, lacking awareness of the means to purify it, he kept it safe. First in the Lake Tower, and later, in the Lacustrine Keep. The Lacustrine keep is a keep built on top of the intake tower of the dam. Access to it is only through the pipe below, or underwater.

Distance of 820 N mass on the lever  = 2.39x10^-2 times depth of lake

The Lacustrine keep regulates the water level of the lake. The magical cloister was permeable to air and water vapor, although not to liquid water, and consequently the lake would have evaporated or leaked out of the cloister field eventually. (When the cloister was active, the lake was dry exterior to the cloister, although moist in immediate proximity to it due to slow leakage of unusually fast-moving water molecules.) It contains