The ability to perceive and manipulate the flow of lifeforce, that fundamental form of energy that flows around the earth
Lifeforce animates all living matter, from animals to vegetable to fungi to bacteria. There's a careful balance to how much lifeforce any living being must have to stay healthy: too little and the being is listless and prone to illness; too much concentrated in one area and the excess will manifest in overgrowth - cancers and bacterial blooms being the most common forms.
A Vivomancer possesses a Sphere of Perception, which is the maximum range within which they can perceive lifeforce in any direction. They also possess a Sphere of Influence, which is the maximum distance from their own skin that they're able to manipulate lifeforce. In the greatest masters their Sphere of Influence extends as far as their Sphere of Perception.
Laurent, however, has not possessed Vivomancy for long. He can crudely pull and shove lifeforce from up to several feet away, but to perform any useful tricks he needs to have several minutes to consider his target's particular distribution of lifeforce and then have his skin within ten centimetres of his target's body. The slower he can move lifeforce, the less unnatural the change will seem to his target.
Traditionally, the most dangerous part of Vivomancy was that the practitioner had to be careful not to use their own body as a sink or source of lifeforce, for ones own force is the easiest to manipulate and yet one is also so close to ones own body that one might not notice problems caused by an excessive ebb and flow until too late.
This was solved by the invention of a metallic alloy able to absorb lifeforce in the same manner as living matter, acting as a battery for the Vivomancer carrying the alloy. In Laurent's case he's forged multiple rings that he wears on both hands. What he's yet to realise is that this metal will slowly leach his own lifeforce if he doesn't keep them full enough of stolen lifeforce to prevent this osmosis. What he does know is that if he fills them too far they'll burst into heatless blue flame, the lifeforce within them finally becoming visible.