Redania's Most Wanted is a Novigrad side quest with suggested level 12. This quest involves looking for clues to the whereabouts of Philippa Eilhart.

Once arriving at the map location of Philippa's hideout, talk to the witch hunters there to discover what they have found so far.

They actually have a "tile" that is required to open the entrance into the hideout.

Entering the Caves

As you jump down the path you'll encounter a purple doorway that is not passable. Look to the right of the door using your witcher senses, and you'll see the place where you can place the recently received tile, to open the door.

As you enter, you'll see another open portal on your left. Enter this portal. Using your witcher senses you'll notice lots of feathers. In the next room you'll encounter some Nekkers. If you're having trouble offing them, remember to apply Necrophage Oil and use Igno, or Yrden, or Quen. The caves are dark -- use the Cat potion or a torch to help find the way.

Next, you will encounter a portal doorway that has become inactive, but you can activate it by blasting it with your Aard sign. Enter the newly opened portal.

In this next room, you'll see a frightened witch hunter. He is holding a power cell crystal, which you can convince him to give you if you are high enough level. If not, you can simply fist-fight him for it. In the next room, use the power cell, and then Aard sign, to open the next portal.

In the following room, you'll encounter more Nekkers. The next room has another inactive portal, and requires another power cell, which Nekkers have taken. You'll need to backtrack slightly and head down a path to kill some more Nekkers to find another power cell. Use the recently obtained power cell to open the portal previously mentioned.

Boss Battle: Ifrit

In the next room, you will encounter Ifrit which is a Fire Elemental. Elemental's are weak to Dimeritium Bombs and Elementa Oil. Be sure to roll out of the way of the creature's fire attacks and you'll be able to beat it pretty quickly. Using your witcher senses, you should explore the caves to find some boots, as well as more feathers. You can use Aard to blast the stalgmites around here, but it won't reveal any hidden passages.

At the end of the large cave, you'll find a room which Philippa appears to have stayed at recently, but is not there anymore. Keep searching the room, and you'll eventually discover two quest items: a crystal from a megascope (Polished crystal), which is behind a pillar, and close to a fiery blaze, as well as agate. 

Witch Hunters

You can now exit the area and go back to the entrance. As soon as you exit the area however, the witch hunters will ask you what you found. You can lie to them or tell them that you found a Megascope Crystal. When you do that, they want what you have found, so they can take the credit.

You have two choices now:

1. Give the witch hunters the Megascope Crystal. They will take it to Radovid and take credit. You will learn nothing.
2. Deny the hunters the crystal and they will fight you. Take them out and keep the crystal. 

Three Choices

You can now continue the quest, but you have to make a big decision. Who will get the Megascope Crystal? 

Here are the choices:

1. Show it to Triss: If you have not yet completed the Now or Never quest, you can head back to Novigrad and meet up with Triss. This will make her very happy and reveal a hidden message about Philippa's quest. After showing it to her, you can decide whether you want her to keep it or bring it to Radovid. 
2. Show it to Yennefer: You can hold on to the item and continue your main quest line until after you're done with Nameless, as well as the Skellige secondary quest: The Last Wish. Note: you have to do this before the main quest Isle of Mists. Take the crystal ot Yennefer and reveal the hidden message.  
3. Give it to Radovid: You can give the Megascope to Radovid. Return to him and he will give it to his mage prisoners. You'll get a reward.
4. Lie to everyone: Finally, if you lied to the witch hungers about the item and then go to Radovid and lie to him as well, he'll be angry at you but won't get the crystal. However, that means you can no longer choose the options to show it to Triss or Yennefer. 

Head back to Radovid to complete the quest. He will ask you about Philippa. If you watched the hidden message, you now have two possible answers -- both lead to the same result. You're getting stiffed for the reward. Alternatively, if you kept the crystal and hand it over (and don't feel about it), you can get some cash.