The suffix -(y)En can be used for relativizing:

The subject of the relativized verb (which you've already seen)
The possessor of a genitive-possesive constituent
The possessed of a genitive-possesive constituent

An example of a genitive-possesive constituent is Evin kapısı, where ev is the possessor and kapı is the possessed part of the constituent. You can see this because ev has a genetive case suffix, whereas kapı has a possessive case suffix.

In the table below, the possessor of a genitive-possesive constituent is relativized.

Full sentence	Relativized version
Kamil'in annesi öldü. Kamil's mother died.	Annesi ölen Kamil Kamil whose mother died
Evin kapısı kırıldı. The door of the house is broken.	Kapısı kırılan ev The house whose door is broken

As you can see above, when relativizing the possessor of a genitive-possesive phrase, you remove the genitive case suffix from the possessor and you move the possessor to the end of the clause. The posessed part of the genitive-possesive constituent doesn't change, and the verb is placed after it, receiving the relativizing suffix -(y)En.

In the table below, the possessed part of a genitive-possesive constituent is relativized.

Full sentence	Relativized version
Can'ın ayağı çatladı. Can's foot is fractured.	Can'ın çatlayan ayağı Can's fractured foot
Telefonunuzun kılıfı pahalıymış. Your phone's case is expensive (apparently).	Telefonunuzun pahalı olan kılıfı Your phone's case which is expensive

As you can see above, when relativizing the possessed part of a genitive-possesive phrase, you move the possessed part to the end of the clause, and the verb receives the relativizing suffix -(y)En.

Usages of kılıf