copied from Taiboo from Reddit :
Event duration: 15th December ~ 25th December
During the event, event quests will appear at the terminal. The story quests will be unlocked on a daily basis. You will be able to begin collecting the event items after clearing the 1st chapter. Challenge the event quests, collect the "Netherworld Sand", and claim presents from Altera The Sun(ta). After clearing the 8th story chapter, you will be able to exchange for the "War God's Exchange Coupon" and Altera The Sun(ta) will officially join.
All main quests up until Babylonia will be at 1/4 AP from 13th Dec ~ 25th Dec.
"Netherworld Sand" will be used to roll for presents in the raffle box.
Main prize:
1st ~ 4th boxes: Altera ascension material "Santa Beard"
5th box: War God's Exchange Coupon
6th box: Legendary Lore
7th ~ 10th box: 4★ ATK Fou
Event items in every box:
Crystal Wreath (to be used to exchange for more copies of Altera The Sun(ta))
Homunculus Babies
Ghost Lanterns
Magecraft Spinal Fluid
World Tree Seeds
Skill Stones of all classes (blue, yellow & red)
4★ and 3★ EXP
Bronze, silver, gold apples
Mana prisms, FP & QP
How to Play:
The bonus Servants will increase the drops for "Snow Baby", "Corpse Bell" and "Hollow Candle".
Exchange for the 5★ event CE Merry Sheep in the shop to increase "Netherworld Sand" drops.
Equip the gacha 5★ CE Starry Nights for "Snow Baby" drops, 4★ CE Party Time for "Corpse Bell" drops and the 3★ CE Snow Pirates for "Hollow Candle" drops.
Event Shop Items:
Crystal Wreath:
4★ Altera The Sun(ta) (for NP strengthening), exchangeable after she officially joins
Sacred Divine Wine
Cursed Cholectocyst
QP
Snow Baby:
5★ Merry Sheep
Demon God Heart
Blood Tearstone
Sealed Demon Lamp
Lancer, Caster & Berserker monuments
4★ HP Fou
QP
Corpse Bell:
5★ Merry Sheep
Primordial Fur
Dull Magatama
Lancer, Caster & Berserker pieces
4★ HP Fou
Snow Baby
Hollow Candle:
5★ Merry Sheep
Meteorite Horseshoes
Fool's Chains
Night Lamenting Iron Stakes
4★ HP Fou
3★ HP Fou
3★ ATK Fou
Corpse Bell
Netherworld Sand:
QP
Event CEs:
Event shop CE -
5★ Merry Sheep: Quick 8% up & Arts 8% up & 3 stars/turn and +1 to "Netherworld Sand" drop.
Event gacha CE -
5★ Starry Nights: Buster 10% up & critical 10% up & start with NP 40% charge and +1 to "Snow Baby" drop
4★ Party Time: Quick 8% up & Buster 8% up & critical 5% up and +1 to "Corpse Bell" drop
3★ Snow Pirates: 1 time dodge & 1 star/turn and +1 to "Hollow Candle" drop
Quality of Life changes:
You can now mark Servants and CEs as "Select", which gives them priority to appear when browsing your inventory.
Bonus drops for the respective Servants can now be seen on the party formation screen, on their portrait.
You can now zoom in on Servant and CE artwork.
If debuff effects are prevented by debuff resistance, it will now show as "Guard" instead of "Miss". "Miss" will still remain when the debuff fails because of its own chances.
In battle, invincibility, invincibility pierce, dodge and sure-hit now display different visual effects upon landing a hit.
Miscellanous notes:
Challenge quest requires clearing final singularity, reward is one (1) lore.
You can now set temporary Servants as your Favourite even if they have not officially joined.
Altera The Sun(ta)'s profile:
Look up at the sky. Those who bear a prayer for the Holy Night in their hearts should be able to see. The single streak of rainbow that dashes through the night sky. The flock of sheep that go "Baa, baa". And clad in that daring outfit, who should be feeling a bit chilly, the form of that beautiful Santa...!
Her name is Altera The Sun(ta).
Because of a miraculous destiny, the Great King of the West, the Sword Princess who bears the Emblem of the Stars, has awoken to her mission as Santa Claus. She is the reliable third-generation Christmas Heroic Spirit that has not bowed to the poor condition (fever) she has experienced for the first time.
This time, as the Great King, there are countless nomadic cavalry following after as per the stories... but they all seem to be in the form of beloved sheep (for Space-use, though). Why's that.
The rainbow bridge that dashes through the Netherworld. The tan and crimson Santa appears, fluffily!