Constitution is a skill that every Avalonian starts with. It is your ability to defend against magical attack. Many of the guilds use enchantment as their prime means of offensive action, and a high constitution will enable you to have inherent resistance to their attempted attack.
Magical defence can be augmented by many factors. Magical resistance is one such and it can be heightened to augment your effective Constitution versus magical attacks and, by enemy action, weakened to reduce your effective Constitution in the same interaction.
Precise innate Constitution and magical resistance effects:
Maximum heightened resistance is reached at 200% of norm. This increases Constitution in defence by 25%.
At 150% to 199% your Constitution in magical defence goes up by 20%.
At 101% to 149% your defence is increased by 10%.
At 50% to 99% your Constitution is reduced by a tenth of the resistance percentage, e..g. at 60% you lose 6%.
Below 50% your defensive Constitution versus magic is reduced to that resistance percentage, e..g. at 5% you will defend with an innate Constitution of only 5% the norm.
Resistance of zero will remove your Constitutional defence as a factor entirely.