walaikum salam warahmatullah
In my opinion lying is prohibited even when joking, so fictional stories are lies.
Lying is an evil characteristic, which all religions and systems of ethics warn against and which man's innate common sense (fitrah) agrees that it is wrong. Those who possess chivalry and sound reason also say this.
Truthfulness is one of the pillars on which the moral survival of the world depends. It is the foundation of praiseworthy characteristics, the cornerstone of Prophet hood, the result of taqwa. Were it not for truthfulness, the rulings of all divinely revealed laws would collapse. Acquiring the characteristic of lying is akin to shedding one's humanity, because speaking is an exclusively human trait.
Our purely monotheistic sharee'ah warns against lying in the Qur'an and Sunnah, and there is consensus that it is haram. The liar will have bad consequences in this world and in the next.
According to sharee'ah, lying is not permitted except in specific cases which do not lead to loss of rights, bloodshed or slander of people's honour, etc., but which concern saving lives, reconciling between people, or creating love between husband and wife.
There is never a day or a moment when sharee'ah permits a person to tell lies or to say whatever he wants based on lies. One of the things that have become widespread among the common people is the so called "April Fool's Day", where they claim that the first day of the fourth solar month is a day when lying is permitted without being subject to any shar'i guidelines. Many bad things may result from this, some of which we will mention below.
The prohibition of lying in Islam:
1. Allah says (interpretation of the meaning):"It is only those who believe not in the Ayaat (proofs, evidences, verses, lessons, signs, revelations, etc.) of Allah, who fabricate falsehood, and it is they who are liars." [al-Nahl 16:105]
Ibn Kathir said: 'Then Allah tells us that His Messenger (peace and blessings of Allah be upon him) is not a fabricator or a liar, because only the most evil of people who do not believe in the signs of Allah, the kuffar and heretics who are known amongst the people for telling lies, tell lies about Allah and His Messenger (peace and blessings of Allah be upon him).
The Messenger Muhammad (peace and blessings of Allah be upon him) was the most truthful of people, the most righteous, and the most perfect in knowledge, action, faith and certain belief. He was known amongst his people for his truthfulness; none of them doubted that, to such an extent that he was known amongst them as 'al-Ameen (the trustworthy) Muhammad.
Hence when Heraclius, the ruler of Rome, asked Abu Sufyaan about the characteristics of the Messenger of Allah (peace and blessings of Allah be upon him), one of the things he asked was, 'Did you ever accuse him of lying before he said what he said?' He said, 'No.' Heraclius said: 'If he refrained from telling lies about people he would not go and tell lies about Allah.' (Tafsir Ibn Kathir, 2/588)
2. It was narrated from Abu Hurayrah that the Prophet (peace and blessings of Allah be upon him) said: "The signs of the hypocrite are three: when he speaks, he lies; when he makes a promise, he breaks it; and when he is entrusted with something, he betrays that trust." (Narrated by al-Bukhari, 33; Muslim, 59). Al-Nawawi said: "What the commentators and most of the scholars said - which is correct - is that this means that these characteristics are the characteristics of hypocrisy, and the one who has these characteristics resembles the hypocrites in this sense.'
The words of the Prophet (peace and blessings of Allah be upon him), 'he is a pure hypocrite' mean that he strongly resembles the hypocrites because of these characteristics. Some of the scholars said: this is concerning one in whom these characteristics predominate; one in whom these characteristics rarely appear is not included in this.
This is the favoured view concerning the meaning of this hadith. Imam Abu 'Eesa al-Tirmidhi (may Allah be pleased with him) narrated this meaning from the scholars and said: 'The meaning of this according to the scholars is hypocrisy in one's actions.'" (Sharh Muslim, 2/46-47)
Lying in jest:
Some people think that it is permissible to tell lies if it is in jest. This is the excuse that they use for telling lies on April 1st or on other days. This is wrong. There is no basis for this in the pure sharee'ah. Lying is haram whether the one who does it is joking or is serious. Lying in jest is haram just like other kinds of lies.
It was narrated that Ibn 'Umar said: "The Prophet (peace and blessings of Allah be upon him) said, 'I joke, but I speak nothing but the truth.'" (narrated by al-Tabaraani in al-Mu'jam al-Kabeer, 12/391). This hadith was classed as hasan by al-Haythami in Majma' al-Zawaa'id, 8/89, and classed as Sahih by Shaikh al-Albani (may Allah have mercy on him) in Sahih al-Jaami, 2494)
'Abd al-Rahmaan ibn Abi Laylaa said: the companions of the Messenger of Allah (peace and blessings of Allah be upon him) told us that they were traveling with the Messenger of Allah (peace and blessings of Allah be upon him). A man among them fell asleep and some of them went and took his arrows. When the man woke up, he got alarmed (because his arrows were missing) and the people laughed.
The Prophet (peace and blessings of Allah be upon him) said, "What are you laughing at?" They said, "Nothing, except that we took the arrows and he got alarmed." The Messenger of Allah (peace and blessings of Allah be upon him) said: "It is not permissible for a Muslim to frighten another Muslim." (Narrated by Abu Dawood, 5004; Ahmad, 22555 - this version narrated by Ahmad). This hadith was classed as sahih by al-Albani in Sahih al-Jaami', 7658)
It was narrated from 'Abd-Allah ibn al-Saa'ib ibn Yazid from his father from his grandfather that he heard the Messenger of Allah (peace and blessings of Allah be upon him) say: "None of you should take the belongings of his brother, whether in jest or otherwise. Whoever has taken the stick of his brother, let him return it." (Narrated by Abu Dawood, 5003; al-Tirmidhi, 2160 in brief. The hadith was classed as hasan by Shaikh al-Albani in Sahih al-Jaami', 7578)
Lying when playing with children:
We must beware of lying when playing with children, because that will be written down (in the record of deeds of) the one who does that. The Messenger of Allah (peace and blessings of Allah be upon him) warned against doing that. It was narrated that 'Abd-Allah ibn 'Aamir (may Allah be pleased with him) said: "My mother called me one day when the Messenger of Allah (peace and blessings of Allah be upon him) was sitting in our house. She said, 'Come here, and I will give you (something).' The Messenger of Allah (peace and blessings of Allah be upon him) said, 'What do you want to give him?' She said, 'I will give him a date.' The Messenger of Allah (peace and blessings of Allah be upon him) said: 'If you had not given him something, you would have been lying.'"
And it was narrated that Abu Hurayrah said: "The Messenger of Allah (peace and blessings of Allah be upon him) said: 'Whoever says to a child, 'Come here and take this,' then does not give him something, this is counted as a lie.'" (Narrated by Abu Dawood, 4991. This hadith was classed as hasan by Shaikh al-'Albani in Sahih al-Jaami', 1319.)
Lying to make people laugh:
It was narrated that Mu'awiyah ibn Haydah said: "I heard the Prophet (peace and blessings of Allah be upon him) say: 'Woe to the one who talks to make the people laugh and tells lies, woe to him, woe to him.'" (narrated by al-Tirmidhi, 235. He said: this is a hasan hadith. Also narrated by Abu Dawood, 4990.
I would recommend books like The Quran, The Prophets Biography, Du'a the Weapon of a Believer, Quran Atlas, or any other really good book to do with the History of Islam, Islam in General and about Heroes of Islam,for example, Ibn Taymiyyah.