RUINED TEMPLE
If you took Brother Genitivi with you, he will open the temple door and remain at the temple entrance doing research, so you do not have to protect him.
The temple ruins have a fairly straightforward layout, although keys are required to progress. There are many Cultists in the halls, some of them mages. The first passage to the left (Ruined Temple Map, L1) leads to a small group of unarmed Cultists. In a cabinet in this room, the Warden will find a Taper that will be necessary later on. While the bookshelf in the same room yields the Discovering Dragon's Blood: Potions, Tinctures, and Spicy Sauces tome (Wynn's Gift).
An Adventurer's corpse in the same passage yields a letter for the Unbound quest.
Scroll of Banastor
The passage to the right (Ruined Temple Map, L2) known as the South Eastern Chamber leads to a locked room (Ruined Temple Map, L3) (requires a key, see below), and a room beyond it (Ruined Temple Map, L4) in a square room holding a scroll for The Scrolls of Banastor quest, and a set of Ancient Encrypted Scrolls for the Forgotten Verses quest.
Forgotten Versus Side Quest
To find the key that opens the locked door in the South Eastern Chamber, the Warden should head up the first flight of stairs (Ruined Temple Map, L5) which is guarded by Cultists, open the door to the left (Ruined Temple Map, L6) watch out for Bronto just past this door, and reach the Cultist Chambers (Ruined Temple Map, L7). A large number of Cultists awaits the group in this area (watch out for traps as well), so it is recommended to proceed with caution, trying to pull small groups to the companions.
Another scroll for The Scrolls of Banastor quest is found in the first room of the Cultist Chambers.
The last room (Ruined Temple Map, L8) in these Chambers yields the South-East Chamber Key (which will unlock the matching door), a Black Pearl (which, together with the Taper mentioned above, will be required to progress), and Chasind Sack Mead.
Once the key is acquired, the group can return downstairs to unlock the South-East Chamber (Ruined Temple Map, L3) but watch out a new group of Cultists will stand in the way. Within that Chamber, a chest holds the Main Hall Key which will unlock the door at the top of the stairs (Ruined Temple Map, L9). Back in the entrance hall, check the eastward door (Ruined Temple Map, L10) that leads to a wavy corridor at the top of the stairs to find the Armor of Diligence on the corpse of a fallen knight.
Beyond the Main Hall door lies a large room with more stairs (Ruined Temple Map, L11). A Cultist mage and a couple of Cultist archers are guarding the area. The mage can be pulled by himself (or simply use stealth to get near him and backstab him) but once the party approaches the stairs or attacks the archers, a pair of ash wraiths appear on the sides. At the top of the stairs, the party will find a Brazier and a door. Lighting the Brazier requires the Taper and the Black Pearl and yields Codex entry: The Holy Brazier. Beyond the door lies a room with archers and many traps. One point of Stealth is enough to safely disarm all the traps without being seen by the enemy, so the Warden's party can take on the archers without any obstacles.
The next door is locked until the Brazier is lit. Opening the door leads to a room (Ruined Temple Map, L12) with a circular platform in the middle, and a statue along the North wall. An Auto-Save will take place upon entering this room, and shortly thereafter a Lieutenant-grade ash wraith will appear. The spirit also gets some help from two pairs of Cultists awaiting right outside the room (a reaver and a mage on one side, a reaver and an archer on the other). After the fight, the Warden can open the Iron Chest to find a pair of Ancient Elven Gloves.
The party can continue along the eastern (Ruined Temple Map, L13) or western (Ruined Temple Map, L14) wings at this point they lead to the same location, or clear them both for loot and experience. Both wings include Cultists and traps. The west wing can be tougher if the Warden opens a collection of chests, releasing ash wraiths. If Alistair, Leliana, Morrigan, Oghren, Sten, Wynne, or Zevran are in the party, one will comment on the statue in the topmost chamber on either side just before the stairs.
Eventually, the group will reach stairs (Ruined Temple Map, L15) leading up to the Wyrmling Lair.
Wyrmling Lair/Mountainside Caverns
Once you reach the Wyrmling Lair, you have effectively left the temple and are now walking through several underground caverns. In addition to more Cultists and their traps, there are drakes and dragonlings (which sometimes spawn behind, or all-around the party) here in this area.
From the second room in the cavern portion (the first place where there is a choice of directions), the party can follow an eastward path that leads to a room with cages for pigs and goats, sometimes referred to as The Feeding Chamber (Ruined Temple Caves Map, L1). Two drakes on PS3 and Xbox, or three drakes on PC, will spawn and attack once the group is inside the area. They will drop a Drake Scale each, valid for the Drake Scale Armor quest. This will be a very tough fight, as each of them is lieutenant level and they possess the extremely dangerous Overwhelm ability. In the same room, the Warden can find a Stone Warrior Statuette (Alistair's Gift) in a Pile of Dragon Filth.
The other path (Ruined Temple Caves Map, L2) from the second room leads to a farther split. The party can follow a northeast (Ruined Temple Caves Map, L3) or a northwest (Ruined Temple Caves Map, L4) direction.
The northeast path leads to a couple of rooms sometimes referred to as the Hatchery (Ruined Temple Caves Map, L5), due to the various dragon eggs lying around. First, the party will enter a large room with columns, and a flight of stairs. On top of the stairs, a Cultist overseer awaits. Before the stairs, Cultist reavers and archers stand guard, and just to add a bit of spice, there are a couple of traps just at the bottom of the stairs. The party may try and pull the groups at the bottom of the stairs before engaging the overseer. Once the overseer enters the fight, two pairs of drakes will spawn and join the fray accompanied by a couple of dragonlings.
The drakes will drop Drake Scales, and the overseer will drop a Spellweaver.
Note: that one pair of drakes will only spawn if you go up the stairs. You can thus make the fight easier by pulling the overseer down, wiping out the forces, then fighting the last two drakes separately.
One more room lies to the northeast (Ruined Temple Caves Map, L6) of this encounter, where another drake awaits, aided by a Cultist mage, an archer, and a hidden assassin. A book on one of the tables in this room yields the Codex entry: The First Blight: Chapter 3 and a Dragon Egg yields Lifedrinker (necklace for Blood Mages) and the matching Codex entry: The Life Drinker.
An easy way to get through the caverns is to bring a character that has the stealth ability, and sneak past every enemy, and then speak to Kolgrim. Doing so warps your party to the end of the cavern, though you still need to fight the enemies should you ever return, even if you side with Kolgrim. Oddly enough, there seems to be no negative reaction from the cult if you kill cultists after joining them.
This next part can go one of two ways, I am putting in the Kolgrim's Offer, and also the quest you shall receive if you accept their offer.
Kolgrim
Kolgrim’s Offer
The northwest (Ruined Temple Caves Map, L4) path leads to a room with a few Cultists (and their traps), and eventually a large cavern (Ruined Temple Caves Map, L7) where the leader of the cult, Father Kolgrim, awaits. As the Warden approaches, he will initiate a dialogue, revealing more details about the Cult that the party has been fighting since entering Haven. The Disciples of Andraste originally used to guard her ashes - however, they claim the prophet has since risen from the dead and the cult acts as her guard.
If, during the conversation, the Warden mentions that the real Andraste is dead and/or that the one the Cult is following now is a fraud, Kolgrim and his aides will attack the party. Otherwise, the dialogue may lead to Kolgrim offering a task for the Warden to make up for all the damage inflicted so far to the Cult (and to become a brother/sister to the Cult): take a vial of Andraste's blood, find a way around the Guardian protecting the ashes, and pour the blood into the ashes.
Accepting Kolgrim's task and defiling the ashes is the only way to unlock the Reaver specialization and obtain the Sacrilegious achievement. But if you have saved before you initiate the conversation, you can unlock the achievement and specialization and then revert to the save. Your rewards will be preserved even if you then choose another course of action.