To get clear,
any skill cast requires a source.
Ok, you never sent damage, you increase or decrease health.
By decreasing the health, will be shown that visual color (poison warning).
Find the same function that increase the health constantly and you'll have it.
Through packets, you'll have to check
SERVER_ENTITY_STATUS_UPDATE = 0x3057 which handles bad status and increasing/decreasing HP and MP, but I'm pretty sure you'll have sync issues with GS/database if you manipulate it by yourself.
I'm not PK actually, but I got PK just to confirm it.