The Medrash (Shmos Rabba 9,23) explains that the Ten Makkos were divided into four sections. The first three of דם צפרדע כנים which were on the earth were brought about by Aharon. The ברד ארבה וחשך which plagued the air and atmosphere were brought about by Moshe Rabbeinu. The ערוב דבר מכת בכורות were brought about by Hashem. The remaining plague of שחין was brought about by all three. As the Medrash explains Hashem said to Moshe and Aharon take for yourselves two handfuls of ashes from a furnace and Aharon's ashes were placed miraculously into Moshe's hands as well. Moshe then threw the ashes heavenwards and Hashem transformed the ashes into Shechin - leprosy in the heavens and then it descended upon the land of Mitzrayim as a dust falling on the entire population and its livestock.
Why were the plagues were divided amongst three implementers and why was the plague of Shecin shared by all three? The Minchas Ani answers that Mitzrayim sinned against the earthly body of Am Yisrael by their severe bondage that was forced upon them. They were therefore punished with the first three Makkos which were on the earth. They threw their children in the Nile brought to them punishment to them with ברד ארבה חושך which struck in the atmosphere which is equally close both to the heaven (the neshmah) and the earth (the body). The third sin that they rebelled against Hashem was punished with ערוב דבר מכת בכורות executed by Hashem alone.
The question that we have to deal with is why did the plague of Shechin come about specifically by all three of them? He answers that Shechin which is a type of tzaraas is given as the punishment for Lashon Harah (Vayikrah Rabba 17,3). Concerning every case of Lashon Harah (Eruvin 15b) Chazal say that the minimum of two people sin, the one who speaks Lashon Harah and the one who accepts it as true. The one who speaks Lashon Harah produces a severe aveirah in that it ascends all the way up to the Kisay Hakavod.
This explains why the Makkah of Shechin was set aside with its own category brought about by all three Moshe, Aharon, and Hashem. This was in order to convey the severity of the aveirah that it effects directly the one speaking, the listener, and Hashem. The reason why Mitzrayim was struck with Shechin was because they spoke Lashon Harah on Am Yisrael. They claimed that the Jewish nation were going to make a mutiny against them הבה נתחכמה לו ונוסף גם הוא על שונאינו ונלחם בנו ועלו מן הארץ . The passuk says וירעו אותנו המצריים which is translated to mean they made us look bad in their eyes by fabricating conspiracy theories and fake narratives. The name פרעה which consist of the two words פה רע evil mouth, convey that it was him who spread these false accusations on Am Yisrael.
This is the reason why the first sign demonstrated to Pharaoh by Moshe and Aharon was his staff turning into a snake. The Primeval Snake was the first being to speak Lashon Harah against Hashem. He said to Chavah that Hashem ate from the Aitz Hadaas in order to enable himself to create the world. He therefore forbade you from eating from it because He fears competition from you. All Lashon Harah stems from this episode. Even the word שחין possesses the letters of נחש. Moshe with his staff transforming into a snake was telling Pharaoh that you are being punished because you as the snake have spoken Lashon Harah on Am Yisrael. Your empire will collapse without anything to stand on just like the Nachash lost its feet and erect stance.
However, Pharaoh was not quick to acquiesce for he reasoned that Yisrael also speak Lashon Harah. You Moshe, fled from Mitzrayim for fear of death due to the Lashon Harah of Jewish informers. Yisrael could not get themselves freed from bondage if they themselves possessed the same flaw which they did. As Chazal say on the passukאכן נודע הדבר that Moshe after his confrontation with Dasan and Aviran who reported his killing of the Mitzri to Pharaoh commented and said now it is revealed to me the reason why Yisrael is still in bondage. It is because they speak Lashon Harah and thereby lack achdus amongst them which is presently restraining their exodus from Mitzrayim.
Little did Pharaoh know how Yosef came down to Mitzrayim as a slave for otherwise he would have become even more adamant in refusing Yisrael to leave the country. It was a story that incriminates his brothers with the aveirah of Lashon Harah towards Yosef that he conspsired to remove them from being a part Klal Yisrael and to lose favor in the eyes of their father.
Therefore if their offspring still speak Lashon Harah today, they have not corrected their shortcoming which brought them down to Mitzrayim in the first place, However Pharaoh did not know that Yisrael already did teshuvah on this particular aveirah and therefore they are truly redeemable from Mitzrayim. Many proofs have been brought to attest to their repair of Lashon Harah. The Medrash says that because of four reasons Am Yisrael were redeemed, they did not change their names, their clothing, their language and did not speak Lashon Harah. Secondly, during the night of Maakas Bechoros there was the phenomenon that no dogs barked even thought there was not a house in which there did not occur a death which naturally triggers dogs to bark. Chazal Pesachim 118 say that one who speaks Lashon Harah should be thrown to the dogs. If it was true that Yidden have not yet corrected this particular aveirah, then the dogs surely would have barked. Thirdly, Hashem told the Yidden to borrow from the Egyptians golden and silver vessels to hold for a year until they leave and then take them with them during their exodus from Egypt. If Yisrael were so immersed in Lashon Harah neighbors would have told the Egyptian lenders that in reality they do not any intention to return all the riches back to them. Yet we find that Yisrael were described as leaving with a great wealth which demonstrated that no one spoke Lashon Harah on their neighbors.
This is the reason why we find by the plague of Shechin representing the aveirah of Lashon Harah, a novel detail that was absent by the first five Makos. By Shechin Hashem hardened his heart ויחזק ה' אל לב פרעהin contrast to the previous plagues where we find that he himself hardened his own heart (see Rishonim). Rav Chisda says that one who speaks Lashon Harah removes the Shechina's presence from one's midst (Sota 42).This goes hand in hand with what we mentioned above that the sin of Lashon Harah causes a direct affront to the dignity of Hashem's Shechina residing at the Kisay Hakavod.
This will explain why the Pirkei D'Rav Eliezer writes that by the severity of Shechin we find that Pharaoh was never cured from it. Pharaoh the פה רע was the source of spreading his Lashon Harah concerning millions of Am Yisrael and spreading it to the entire Mitzrayim population. The massive quantity made it impossible to reverse and correct his evil wrong doing. Today with modern technology unforturnately a text or post can reach millions very quickly and cannot be retrieved once the damage is done.
Furthermore, פרעה was the embodiment of the Nachash and he is called תנין snake (Yechezkail 29,3) Chazal say
א"ל הקב"ה אתה ספרת לה"ר סופך ללקות בצרעת שנאמר שם ויאמר ה' אלהים אל הנחש כי עשית זאת ארור אתה מכל הבהמה במה אררו בצרעת שנאמר ויקרא י"ג כי צרעת ממארת היא אמר רב הונא בשם ריב"ל הסלעים שהן על הנחש היא צרעתו ולא עוד אלא כל בעלי מומין מתרפאין בעולם הבא והנחש אינו מתרפא שנאמר בראשית ג ארור אתה מכל הבהמה מכאן שהכל מתרפאין והוא אינו מתרפא
Hashem cursed the Nachash with tzaraas. Rav Hunah said that is why the skin of the snake appears like tzaraas. All those that possess blemishes will have them cured in the future except for the snake. According to this Chazal we can say that since פרעה embodied the snake with Lashon Harah, his punishment of שחין will be everlasting just like the Nachash. Therefore one might have thought that since the punishment of Shechin will not go away, Pharaoh would do teshuvah and wouldn't become stubborn as before in order to rid himself of the lingering tzaraas and then there would be no further need for the four future Makkos. Therefore Hashem hardened his heart in order to smite Mitzrayim with four more Makkos in order to build up to Ten correlating to the Ten sayings through which the world came into existence. Even Yaakov Avinu davened to Hashem to save him from speaking Lashon Harah as the Medrash says on this passuk אם יהיה אלקים עמדי ושמרני בדרך הזה אשר אנכי הולך (בראשית כח כ)).
The Chofetz Chaim explains
שלכך ביקש יעקב על עוון לשון הרע בפרט כיון שהוא הלך ללבן שהוא מקום סכנה והוא צריך שמירת ה' יתברך ובעוון לשון הרע השכינה מסתלקת לכך ביקש יעקב אם יהיה אלקים עמדי שישמרני שלא אחטא בעוון לשון הרע ואז אזכה שה' יהיה עמדי ושמרני ומובטח אני שאשוב בשלום לבית אבי
Yaakov's going to live and work by Lavan was a grave danger both physically and spiritually. Yaakov davened to be protected from speaking Lashon Harah because only then will Hashem be with him to protect and watch over him and bring him home in peace to his father.
If we speak Lashon Harah then all the nations will spread Lashon Harah on us which places Am Yisrael in daily danger as we have seen an increase of episodes over the last few months of serious and precarious acts of Anti Semitism. Are we doing enough as individuals of Klal Yisrael to stop their big mouths when we allow our own mouths to open up and speak the same lashon hara on our fellow Yidden. You can save the lives of our brethren by doing your part every day. Every day before you lies a choice of either Shechinah or Shechin. One cannot have a loose tongue and expect at the same time to have protection from Hashem. Even Yaakov with his greatest ruchniyus understood this very well and that is why he davened to Hashem to save him from this aveirah.