Astrals (summons) are truly something to behold in Final Fantasy XV. These massive deities are the center piece of the story of the crowned prince, his lineage, and the events unfolding throughout the entire game.
Astrals will assist your party, once you have passed their trials. Two astrals you face, while two give tasks to complete. One Astral, Bahamut, is strictly a story-based summon. He comes at a certain point in the game only, and cannot be summoned any other time.

Summoning an Astral into a fight is very different from titles before. Rather than being able to summon them at will, there is a random chance for them to appear. And not always will you get the diety you think you might. Key factors on when a Astral will appear are party condition, environment, as well as weather. You can go literally dozens of battles, and not once will you get aid from one of these very powerful allies.


If you do happen to find favor with one of the astrals, the screen will darken to a reddish tint, and a heartbeat sound will occur. When the button appears, hold it down for several seconds. Your astral will come into the battle at this time, and do their damage.

Although Astrals provide the heaviest firepower in the game, with wide area effect damage, it does not mean you should call on them every time the chance arises. These deities will only appear on a semi-regular basis, and will only come to Noctus's aid one time during battle. (Scripted appearances of Astrals being the only time you have more than one summon per battle.) What does this mean exactly...? Here is a breakdown of what this means exactly.

Every 10 seconds during battle a "Summon Probability Check" is performed. If the requirements for one or more are met at this check, the appears. If you accept, the Astral comes to your aid. Once this happens, a cool down time is triggered for your summons, before they can reappear.
It would make more sense, if you can get through the battle alone, to not call on them. You do not know when a particularly nasty boss or hunt would require their aid even more. Yes using them against a herd of Sabertusks will grant the grand spectacle of your Astrals appearance....but you might then stumble across a high level Deamon or boss that you wished you could have them then. Use your Astrals wisely.
Below is a list of the Astrals you can summon (and Bahamut), as well as their conditions for summoning and Chapter you receive them in.

	Chapter Acquired
	End of Chapter 04. You will have to face Titan's trial in battle, to win his power.
	Condition Factors to Summon
	The more party members fall in battle, the better chance you have of him coming to your aid.
	Enviromental Factors to summon
	Titan prefers the wide open ranges in the world. Seemingly on the more rock/dirt kind of places. If in the open terrain, he will punch the ground causing a quake. If more closed in, he will throw a massive boulder at the enemies you are fighting.

	Chapter Acquired
	Chapter 05. The entirety of this chapter is facing Ramuh's trial, and winning his favor. You do not have to face Ramuh, as you did with Titan. But the trial is frought with fights and roughness on its own to not have to have a major battle with this thunder giant.
	Condition Factors to Summon
	The longer you remain in battle with the same enemy/ies, the more chance you have of Ramuh coming to make short work of your foes.
	Enviromental Factors to summon
	Ramuh is a anywhere, anytime diety. This includes the interiors of dungeons, and inside access areas. He is bound by no limits provided by the gaming environments. (I would have thought him a diety to appear in rainy weather....but who am I to know?

	Chapter Acquired
	End of Chapter 09. You will fight this sea goddess in a epic battle, when you reach the climax of Chapter 09. It is an amazing, and brutal battle to contend with, so come prepared.
	Condition Factors to Summon
	Leviathan will only come to the party's aid, if Noctus is in danger status. The longer he remains in this status, the better chances you have of the goddess of the sea lending her aid in battle.
	Enviromental Factors to summon
	Since Leviathan is a goddess of the sea, it stands to say she will only appear when you are around large bodies of water. Not only is this the coastal area, but the rivers and major lakes provide the water she requires to come forth.

	Chapter Acquired
	End of CHapter 12. You will spend a good deal of Chapter 12, going through Shiva's trial. But oddly enough, you discover she has been there through most of your travels already. Again, there is no major boss battle to win this beautiful goddesses favor. Just make it through her test, and she will come with you.
	Condition Factors to Summon
	Shiva's appearance does not really have to do with party anything. It has to do with the combinations of probability that the other Astrals will come to your aid instead.
	Enviromental Factors to summon
	Shiva is another anytime, anywhere Astral. She comes to the party's aid in both wide range map areas, and interior dungeon settings.

	Chapter Acquired
	Chapter 14 (Scripted)
	Bahamut will only appear one time in Final Fantasy XV as a summon. With the addition of the Royal Edition you will only get to see this god of gods one other time. Again it is also scripted to be. It is truly sad there is no use of him in battle, because his power is insanely devistating. But if he is indeed the lord of all...he knows when the right time to appear would be.