What is a Seal Quest?
A Seal Quest is basically a set of challenge/raid, where you need to accomplish a set of tasks before the time limit. Each Seal Quest features a bunch of unique monsters with unique abilities, and a specially crafted map of their own. All 4 of the quests are unique on their own way. They can be done in any order except Yuno Seal which can only be accessed once you've finished the other 3 Seal Quests.
How do I go on completing a Seal Quest?
You'll need a party of six members to participate in a quest. A few geared and experienced people, and advanced reading of what each quest entails are recommended to ensure a smooth ride for each quest. You are allowed to dual-client (maximum number of client allowed to enter the Seals) if need be. There are no class restriction or level requirement to join a quest i.e. even a level 1 Novice can participate in the quest. The higher leveled are always recommended as these quests are considered High-End PvM'ing as the goal is to fully utilize the SQIs. You can repeat a quest as many time as you want (to help other people in need for example) with the characters who have already unlocked the seals. Each quest completion is also game account bound i.e. if you did Prontera, Niflheim, Manuk Seals on a High Priest, you can use another character to finish Yuno Seals as long as it's in the same game account. Upon completing any Seal Quest, you'll be rewarded with a random Essence.
An essence is a custom item (ingredient) unique to the server. They are required to make, and bonus an SQI. They're found in game as Essence of [Class], and there's an Essence for each different class amounting to a total of 18 different essence can be found in game. They're a reward for completing a Seal Quest (not only the first time, but every time you finish a quest) and is tradeable/vendable to other players.