I would like to know why my health and mana decrease, I notice that they go down from 1,200 to 1,134 on my character page and it said that it's full, I don't understand.. and my mana was 70 now it's 54.
When you take damage in a duel, your health is affected. You will need to find health wisps (red ones) to regain that health in a combat area, or return to a non-combat area to heal naturally.
When you cast spells in a duel, your mana is used to cast those spells, so it will decrease, and you will either need to find mana wisps, or use a potion to get your mana back.
If you are referring to your MAXIMUM health and mana, many items you equip such as rings or athames boost your health and mana. If you take those items off, your health and mana are no longer boosted and you will see a drop in your maximum amounts.