Recovering the Key, Fateful Decisions, For the Reunion, and Over the Wall are quests in Chapter 14, "In Search of Hope", in Final Fantasy VII Remake. The quests take place in the Sewer System. While helping Leslie Kyle in exchange for information on how to reach the Shinra Building, Cloud Strife, Tifa Lockhart, and Barret Wallace chase after a key to Don Corneo's hideout which was stolen.
This quest is required to complete the sidequest "Corneo's Secret Stash".
Walkthrough[]
When the quest begins, sprint down the linear path, following the Abzu shoat. Defeat the wererats that appear, and then the sahagins shortly after, then use the ladder on the left to the upper level. Crossing the bridge to the left leads to a treasure chest with two tufts of phoenix down, while using the path right underneath a stack of crates leads forward to where a different Abzu shoat is fought. This is a fairly weak enemy which can be defeated in a few character abilities and fire materia spells, but is capable of spinning, meaning melee attacks should not be used against it until it stops. Proceed right over several obstacles and chase the original Abzu shoat to lead to an area with a few more Abzu shots, which once again can be dealt with quickly. Proceed right to chase the original Abzu shoat further, then use the door to the room on the right and defeat a group of Abzu shoats and a sahagin inside. Head through the door.
After defeating the Abzu shoats and sahagin, head through the door following the original Abzu shoat, then continue running right. It will initially wait, before jumping down to the left. Use the ladder on the left and defeat the scissorclaws and blugus, then open the treasure chest left and head down the stairs forward and chase the Abzu shoat, leading to a short cutscene. Following this, as Cloud, walk towards the original Abzu shoat, and then when it escapes, run to chase it.
Eventually, the original Abzu shoat is fought, named the Mischievous Shoat in battle. This enemy is more durable than the other Abzu shoats, but can also be defeated best with fire spells. After dealing damage to it, it will summon several smaller Abzu shoats. Defeat these first, then focus on the Mischievous Shoat with abilities and fire spells.
Protective boots.
After defeating the Mischievous Shoat, a cutscene commences. Following this, head through the door ahead and follow Leslie down a corridor. When a lever appears, pull it for "Corneo's Secret Stash", which can be completed further back by following the same route where the original Abzu shoat was chased by Cloud to; a pair of protective boots are also found in a treasure chest here. After this, climb the ladder and follow Leslie until he heads through a door. The vending machine and bench can be used nearby to heal before running through, as a boss battle follows. The fire materia is useful in the next fight, and pairing it with 
Elemental Materia is also effective on characters who focus on using their abilities. Characters focusing on spellcasting can also pair fire with the HP Absorption Materia as a substitute for healing; this can be particularly useful on Barret as he will often need to stay at high HP. Equipping 
Healing Materia is crucial, and giving one character healing materia paired with Magnify Materia can help significantly. Ifrit should also be equipped. The Time Materia is also useful if possessed.
When ready, follow Leslie through the door. After this is a cutscene, following which a boss battle is fought.
Boss battle[]
Abzu.
The boss battle is fought against Abzu, this time with Abzu shoats backing it up. Abzu has similar melee attacks, which can be avoided, and should be fought with fire spells and character abilities. Crippling its horn can pressure it. The Abzu shoats can be defeated to help focus on Abzu itself. It can also leap to the side and summon water, which should be dodged by hiding behind crates. If a character has time materia, cast 
Haste on strong damage dealers, such as those with powerful character abilities.
After it loses half its HP, it will summon more Abzu shoats. These can be defeated with abilities that hit multiple foes, such as Cloud's Triple Slash, Tifa's Divekick, or Barret's Smackdown to deal with them, though Tifa is also ideal for chasing them due to her movement speed. The tactics are otherwise the same. It is later capable of using Pounce which can stun a character if not dodged, or Triple Charge, which should be guarded against to avoid damage, though Barret can also focus on crippling Abzu's tail to pressure it, and then lay into it with powerful attacks.
When Abzu is defeated, use the door and talk to Leslie, leading to a cutscene. After this, follow him to a ladder, prompting the player to leave the sewers. After this is a cutscene, and Leslie takes the party to Wall Market's Urban Advancement District.
The wall at the back of Wall Market.
In Wall Market, it is ideal to finish any remaining sidequests now. When all are completed, the Letter from the Guardian Angel can be found in Wall Market's Urban Advancement District, close to the sewer exit, along with an Elemental Materia.
To continue to the next quest, head forward towards a wall, and Barret will warn Cloud that they will not be able to return after climbing the wall. Agree to climb the wall to proceed, and after a cutscene, the next quest, "A Broken City", in the chapter "The Day Midgar Stood Still", takes place.
Hard mode tips[]
As with the previous quests, for the first few fights, conserving MP is ideal. Avoid casting spells, use 
Prayer Materia to heal, and pair elemental materia with fire against the sahagins. 
Parry Materia should be used on Tifa for dodging, while Cloud and Barret equip Steadfast Block Materia to mitigate damage, against the Jump attacks.
When fighting Abzu again, fire materia should be equipped, especially if it can be paired with elemental on Cloud and Tifa, or with HP Absorption or MP Absorption Materia on Cloud or Barret. Equipping Barrier Materia with magnify can allow Barrier to be cast on everyone, while Barret uses Lifesaver to keep the party protected. Pray should be sufficient for healing. Once again, time materia is very helpful. Deadly Dodge Materia is also great on Tifa and can be paired with HP Absorption Materia to cure her while attacking the smaller shoats. Cloud and Tifa can also benefit greatly from equipping Transference Module to use Cloud's Ascension or Tifa's Dolphin Flurry more quickly, and Refocus Materia is useful on Barret or Tifa.
Destroy the Abzu shoats as they appear before focusing on Abzu with abilities and spells. It is useful to open the fight with Lifesaver and cast Regen on Barret, and if possible, casting Haste on primary damage dealers is ideal. Against Abzu, Cloud's Counterstance is particularly effective at resisting damage. Abzu has great mobility, but casting Stop can render it vulnerable for 10 seconds to Tifa's Unbridled Strength, Barret's Maximum Fury, and Cloud's Infinity's End, though it should not be used just before the end of one of Abzu's phases.
Abzu is most dangerous in the final phase of the fight, when his leaps towards the party are deadly, and the Abzu shoats' diving tackles can interrupt the fight. Ensure Lifesaver is up for this, use Pray just before the phase, and cast Barrier. Guard against the Abzu shoats and then defeat them while casting spells or using abilities against Abzu in between. Against Abzu, cast Stop after its run is finished. With Transference Module, and with sustained damage throughout the fight, it is likely that Cloud or Tifa can finish the battle with Ascension or Dolphin Flurry.
Defeating Abzu will earn Tifa the Way of the Fist Vol. IX.