When you reach level 21 on open-PVP you will lose the Adventurer's Blessing as this blessing is only available on this game type until then. If you die and go back to level 20 without participating in PVP -you won't get the Adventurer's Blessing back, it's lost forever.
" Adventurer's Blessing: Characters on open PvP game worlds up to level 20 are automatically protected by the Adventurer's Blessing. You can easily see if your character is protected by this blessing by the little ankh shown in your inventory. Further, your character's inventory will be displayed in yellow. If your character dies in a PvP fight being protected by this blessing, your character will not lose any items, experience and skill points. But beware! As soon as your character attacks another character first or reaches level 21 for the first time, the blessing will be lost for good" Source: https://www.tibia.com/gameguides/?subtopic=manual§ion=characters