The Order of the Knights Mercurial

In ages long since past, the first wellspring of mercury breached the surface of the world. Living metal, flowing like water, ended its secret vigil in the bones of the earth. The first Knight Mercurial discovered this wellspring and was enraptured by its unworldly beauty. The shimmering, silver surface called to him, a man true of heart and purpose. It called for him to submerge himself, and join in the chorus of the world. He drowned. His lungs flooded with slippery metal. His flesh and bones dissolved, replaced with quicksilver. His mind left him, lost in the overwhelming symphony he knew now to exist in the bowels of the earth. It returned just as fast, filled with a new vigor and sense of purpose and belonging. He rose out of the pool, baptized by the blood of stones.

Years of meditation followed, unfettered by sleep, food, or desire. Others found him, meditating in the grove, unaware of how much time had truly passed. Most ran and the news quickly spread of this strange creature. Many would try to kill him as a monster. None succeeded. Occasionally, though, a brave, earnest soul would step forward to challenge the Knight. Those came under his tutelage and became apostles of this new knightly order. These Knights traveled here and there, protecting travelers until they found mercurial wellsprings of their own to protect from the selfish and depraved. Centuries passed, and the Knights found themselves in far-flung places around The Continent and beyond. As technology advanced alongside magic, the number of Knights dwindled. Those that remain are the most powerful, the most clever, or the most well hidden. They quest no longer, and instead train alone, in preparation for worthy heirs to take their place so that they may rejoin their brothers and sisters within the bones of the world.

A Knight Mercurial is always found guarding a pool of liquid mercury, filled by a nearby spring that produces the metal. Each is looking for ultimate rest at the hands of someone that they deem worthy to learn from their Order. They will not decline a duel or challenge, but will insist on terms that must be met. Their armor is unique to each Knight, but all bear the heraldry of a caduceus, decorated as they each see fit. Thieves are attacked if seen stealing from the pool. If they are bested in the contest they present, they will impart their teachings to the victor only if they survive the "apotheotic process" of giving themselves over to the mercury.

A handful of these Knights can be found scattered around the Oldwood and have the following (system agnostic) characteristics in common:

Attacks 2x Longsword (1d8)

Initiative +2

AC as Plate

HD 4

Standard Movement

Mercurial Form - The Knight Mercurial can Move through any space at least 1 square inch wide, unhindered. They can escape restraints as an action.

Parry - When damaged, the Knight Mercurial may expend 1 HD and reduce the damage by that amount.

Special Maneuver - Each Knight has their own, specialized technique that they have perfected over countless decades.

from Wikipedia

The Knights

Some Knights are more skilled than others, and thus have more HD.

Sir Argus, the Flowing (5 HD)

Flowing Form -When Sir Argus takes damage, he may choose to spend HD and create an identical clone of himself. He may choose for either the clone or his real body to take the damage. If the clone takes damage, it is destroyed and Sir Argus loses as many hit points as the clone had total. The clones die if Sir Argus dies.

Lady Maya, the Quick

Quicksilver - Lady Maya's base movement speed is increased by 10 units. She has advantage on all Dexterity and Strength related Saves and Checks.

Sir Casmil, Knight Commander (8 HD)

Blood of the World - Sir Casmil appears to be unarmed, but actually wields the content of his mercurial spring. On each of his turns he may chose to do one of the following:

• Regain 1 HD.
• Emulate the technique of a living Knight Mercurial until the end of the next round.
• Impale an adjacent target, dealing 1d4 damage to the target's HD directly if the attack hits.

Knight Totan, the Unmoving (6 HD)

Inertial Bulwark - Knight Totan wields only a tower shield. They decreases their AC by 1 and become immobile each time they take damage, until they willingly move. If hit while immobile, Knight Totan may roll HD. Their shield gains Xd4 damage on the next attack they make each time this ability triggers. 

Lady Prim, the Amalgam

Auric Extraction - When targeted by magic of any kind, Lady Prim may roll HD. If the result is 1-4 then she harmlessly absorbs the spell and retains her HD. If the result is a 5-6, she takes that much damage and suffers no effect from the spell.

Lady Calomel, the Restless

Formless - Lady Calomel has AC as Chain instead of Plate. The first attack that hits her each round instead misses. Once per round, Lady Calomel can transform her longsword into any 1 or 2 handed weapon of her choice as a free action.

Apotheosis

If you fully submerge yourself in a mercurial pool for one hour, make a CON save. If you fail, you die. You are filled with a deep sense of connection to the earth itself, and have a vague understanding of where each mercurial pool is located. Each day for the next 3 days, repeat this save. If you survive, your body becomes silver, malleable and cannot be moved against your will. As an action, you can change your appearance and weight, or sharpen your hand into a bladed weapon. You can squeeze through gaps as small as 1 inch as an action. In order to heal, you must ingest a pint of mercury or avoid heavy activity for 1 week. You only need to eat and drink once a week but failing to do so is lethal.