Limited Time Covenant

Limited Time Covenant or UR Altar is a recurring skip-week event (1st=new, 2nd=none, 3rd=old) that allows players to exchange Diamonds for Oracle Covenant (or scrolls), which in turn can be exchanged for UR unit tokens during the event. 
=Pulls=
Players get one free pull per day. Since Limited Time Covenants last four days, that's four free pulls in total. Beyond that, a single pull from these altars costs 300 and 10 pulls cost 2700. The exact drop table of these pulls is undisclosed and unknown, but 10x pulls seem to yield about 12 scrolls on average and the 1x pull seems to yield less per count than the 10x pull. 
On top of the random drops from the pulls themselves, players can claim certain fixed milestones for total number of pulls during the LTC, as listed in the table below:
Milestones
total pulls	reward	cumulative	total extra per pull
10 	2 	2 	0.2
30 	3 	5 	~0.167
50 	5 	10 	0.2
80 	10 	20 	0.25
120 	25 	45 	0.375
170 	35 	80 	~0.471
=Exchanging Scrolls=
As explained, the altar itself will allow players to pull scrolls. By themselves, these scrolls bring no added value.
However, Limited Time Covenant events include a functionality to exchange the scrolls for UR tokens, on a 1:1 rate. This allows saving up for UR units on multiple altars.
Monthly altar event is expected to feature a newly released UR unit, announced by Sound the Horns, so past-released UR units will probably not be available all the time for exchange. Therefore the main reason for investing in scrolls in advance at this point seems to be for the progress chests the event provides. With currenct launched URs, each UR unit will reappear once every 10 weeks.
=Quests=
Every UR unit features a list of quests that the player may complete for valuable rewards. Completing quests will only be possible during the Limited Time Covenant event. Players may view all progress in Museum Missions and claim their rewards, even after the event has ended.
All quests are related to the specific UR unit featured in the altar. It is unknown if for past units they are expected to return (most likely yes).
Quest 	Reward
Get UR unit to 3⭐ 	800
Get UR unit to 4⭐ 	800
Get UR unit to 5⭐ 	10 UR Unit Tokens
Get UR unit to 6⭐ 	Special Skin
Unit Talent 6000 	500
Unit Talent 10000 	500
Unit Talent 14000 	5
Unit Power 60K 	500
Unit Power 80K 	500
Unit Power 100K 	5
Also completing all quests will reward the player with a special Aura (Champion)/Apparition (Commander), associated with that UR unit.
=Release Order=
==Commanders==
Judicator ➤ Troll ➤ Ice Demon ➤ Evil Sorceress ➤ Regnan Seer ➤ 
Dragon Golem ➤ Venom Spawn ➤ Dark Champion ➤ White Tiger ➤ Mantis 
==Champions (First Gen)==
Dark Elemental ➤ Grim Rider ➤ Black Guard ➤ Quetzalcoatl ➤ Abyss Lord ➤ 
Shaman ➤ Sister ➤ Faceless ➤ Arcane Archer ➤ Sword Dancer ➤ Krait 
==Champions (Second Gen)==
Itinerant Priest ➤ Faun ➤ Shadow Panther ➤ Light Elemental ➤ Forest Guard ➤
Fallen Angel ➤ Arcane Eagle ➤ Emerald Dragon ➤ Rakshasa ➤ Light Dragon ➤ Fate Weaver ➤ Kraken ➤ Alchemist 
==Champions (Third Gen)==
Hell Baron ➤ Falconer ➤ Fury ➤ Diamond Elemental ➤ Moon Doe ➤ Scale Guardian ➤ Lamasu ➤ Fossil Crab ➤ Stranger ➤ Cave Hydra ➤ The Shadowless ➤ Magic Golem ➤
=History=
==2022==
Month 	UR Unit(s)	UR Unit(s)	UR Unit(s)
February-2	 	 	
February-1	 	 	
January-2 	 	 	
January-1 	 	 	
==2021 (Bimonthly Altars)==
With the release of White Tiger, the altar schedule has been changed, featuring the event every 2 weeks now. This leads to an average of 2 altars per month, with some exceptions.
===Primary===
Month 	UR Unit(s)	UR Unit(s)	UR Unit(s)
December-2 	 	 	
November-2*	 	 	
November-1 	 	 	
October-1 	 	 	
September-2	 	 	
August 	 	 	
* Off-schedule due to Anniversary Event
===Secondary===
Month 	UR Unit(s)	UR Unit(s)	UR Unit(s)
December-1 	 	 	
November-2*	 	 	
October-2 	 	 	
September-3	 	 	
September-1	 	 	
==2021 (Single Altar)==
An altar was released every 4 weeks. This mostly lead to 1 altar per month, with a small exception due to how the calendar rolled.
Month 	UR Unit(s)	UR Unit(s)	UR Unit(s)
July 	 	 	
June 	 	 	
May-2 	 	 	
May-1 	 	 	
April 	 	 	
March 	 	 	
February	 	 	
January 	 	 	
=Estimating Cost=
To estimate cost you can use the tool in the spreadsheet here: (provided and maintained by Tilwin). Please note that since we are dealing with chance there is of course a degree of randomness involved.
==Average==
On average (12 dropped on average per 10 pulls), here are the estimated current diamond costs for full unit ranks (assuming no excess scrolls exist in the inventory):
From Rank	To Rank	To Rank	To Rank	To Rank
From Rank	3⭐ 	4⭐ 	5⭐ 	6⭐
None 	16.2K 	35.1K 	64.8K 	110.7K
3⭐ 	- 	21.6K 	48.6K 	91.8K
4⭐ 	- 	- 	29.7K 	70.2K
5⭐ 	- 	- 	- 	40.5K
(notice that this assumes an average so if you get unlucky you may require more pulls, hence more diamonds)
== Tokens for Stars ==
From Rank	To Rank	To Rank	To Rank	To Rank
From Rank	3⭐ 	4⭐ 	5⭐ 	6⭐
None 	80 	180 	330 	530
3⭐ 	- 	100 	250 	450
4⭐ 	- 	- 	150 	350
5⭐ 	- 	- 	- 	200
== Pulling in Multiple Events ==
"Limited Time Covenant" altars are different from Limited Time Recruitment in the sense that instead of pulling tokens of a specific unit, the players get to pull Oracle Scrolls. This means that in theory, multiple events can be taken advantage of to pull for the purpose of the same unit.
This approach works in order to minimize the diamond cost - however please note that this does conflict with tempo since not all UR units are available during all "Limited Time Covenant" events.
In order to minimize the diamonds/scroll ratio, it is most cost efficient to perform exactly 17 x10 pulls each "Limited Time Covenant" event. This leads to the following calculations:
* Diamonds per x10 pull: 2700. Total cost per event: 45,900
* Average: ~12/pull. Total: ~204
* Bonus: 80
* Total per event: ~284
In order to reach a 6⭐ UR, we can also factor in the Museum quests for UR units:
* 10 specific UR tokens (reaching 5⭐ on that UR quest)
* 10 (5 from Talent and 5 from Unit Power quests)
Notice that this relies on an average subjected to randomness.