Negative effects and curses are just like debuffs in other games, which may decrease player's attributes, damage, spead or prevent player from gathering resources, etc.
I'll introduce some negative effects and curses in categories. As my level is low, my knowledge to this game is still limited. So, my introduction will be just limited within what I know.
1. Injury
Player may get injury when defeated by monster whose level is not below his/hers level or killed by players (executioner skill will increase the chance to inflict injury). When injured, player loses HP over time in battle, can't equip the armor at the corresponding possition, and can't gather resources.
2. from resource gathering
Splinter: Player may get splinter when gathering resources. After that, player can't pick resource any longer untill he/she is healed. Splinter can be removed by another player with first aid kit or by yourself with liqueur obtained from your professional master.
Burn: Player has a chance to get burn when picking fire flower. When under burn, player can't gather any resources and lose 5 HPs every 10 seconds in battle.
Mark of death: Player has a chance to get mark of death when picking cuckoo flower. This effect decreases speed and maximum HP, causes player to lose 10 HPs every 10 senconds in battle, and prevent player from gathering resource.
(Players offten take advantage of Burn or Mark of death to do naked hunting, because these effects make them die fast.)
3. from scroll and powder
Player can use scroll and powder in battle to give opponent negative effect. For example, scroll of poison can give enemy poisonous damage over time, powder of retribution can cause enemies to suffer retaliative damage when they attack, which is every useful when fighting powerful enemies like the sentry guards in crystal cave for feats quest, powder of weakness can reduce the damage of enemy for several turns, and such like.
4. from mount
Venomous Bite: give enemy 120 damage in 40 seconds. When you get enough mercenary reputation, you can buy varieties of toxin from npc, which can turn your tiger and zorb into poisonous one, then it can cast venomous bite on enemy in battle.
(Other high level mounts have other skills to cause negative effects.)
5. from pet
Surface Wound: Upon experiencing the effects of this spell, which is cast by a pet, the enemy of its master loses x % of life over 6 seconds, but in total not more than x % of your life level.
Lethargy: Using its skills and abilities, the pet casts a spell that weakens the subsequent physical or magical blow by your opponent by x %.
Curse of Harm: This spell, which is cast by a pet, forces the enemy of its master to pay x % of their health for the damage caused by their subsequent blow.
6. from BoE clan
The clan Bringers of Evil is evil, very evil. Well, its name already tells this. You can find BoE HQ at Eclipse Thicket. Player can join it when reaching LV5. If a player is the member of BoE and gets enough reputation, he/she can use varieties of hexes and ball&chains.
Curse of Hex: from Sickly Hex to Gehenna Hex depending on the reputation a player has. It causes the victim to lose HP continuously in battle. High level hex also decreases the victim's speed and the chance to find rare items.
Ball and Chains: 4 kinds of ball and chains which reduce the victim's speed, the highest level ball and chains also causes the victim to lose HP over time in battle.
7. Lost Soul
Player with 3000 relic seekers reputation can buy Solumir's Mirror in antique shop. It's effect, lost soul is to sever the soul from the body, cause a player to be dead for 2 hours. While under this effect, the dead player can't be resurrected by normal method.