What is the Ketav Meriri?
Let's back up a minute and start with a simple understanding.
In Ha'azinu (Deuteronomy 32:24), God warns of:
מְזֵי רָעָב וּלְחֻמֵי רֶשֶׁף,
What's this "ketev mriri"?
To quote R' Aryeh Kaplan's Living Torah:
[They will be] bloated by famine, consumed by fever, cut down by bitter plague ...
cut down (Rashi; Baaley Tosafoth;Ralbag).Ketev in Hebrew; see Isaiah 28:2, Hosea 13:14, Psalms 91:6. Or, 'plague,' al-chalaf in Arabic (Saadia; Ibn Janach; Ibn Ezra; Bachya; Radak, Sherashim); 'destruction' (Septuagint); 'crushed' (Targum).
bitter plague (Saadia), or 'bad air,' possibly 'malaria' (Ibn Ezra; Ralbag). Meriri in Hebrew. Or, 'unquenchable destruction' (Septuagint); 'robbers' (Rashbam); 'evil spirits,' or, 'bad vapors' (Targum); 'demons' (Sifri; Rashi); or, 'madness' (Tzafenath Paaneach; cf. Sifri on Deuteronomy 21:18).
It's generally known that the Three Weeks are considered an inauspicious time; ask your local rabbi exactly what risky activities you should therefore curtail during this period.
Beyond that, if a good Jewish person assumes that halacha is what made it into the Rambam and Shulchan Aruch, and none of the demon stuff appears there, so what to make of the aggadic spooky stuff in the Gemara? I'm sure there's a deeper meaning, but for now, Teiku.
Or best of all, keep the Torah and mitzvos, so we don't get any of the punishments warned about.
Rav Yitzchak Meir Morgenstern in Nishmatin Chaditin on Vayishlach 5773 section 40 writes (my own translation)
Ketav Meriri has power between sun and shade, and a person must be careful not to walk in an area where the two meet (meaning on the boarder between a sunny and shaded area). The Baal HaSulam explains that day represents torah and mitzvos and shade represents aveiros. But, there are things that are allowable but not commandments (inyanei reshus) that fall in the middle [which is the husk of Nogah-Glow], and a person needs to know that his whole role in this world is to assert the Godliness in those permissible things. One who does not do so increases the husks of evil in the world, and that is called Ketav Meriri, which he thereby gives permission to have dominion over him (meaning over the person who has not sanctified the permissible things).
It is a dangerous spirit that is active in between 17 of Tammuz and Tishabav (according to one opinion) from the 4th hour until the ninth hour of the day(summer time around 11-2). Which is one of the reasons to not do things that are potentially dangerous during the three weeks. This includes things like: hitting children(beis yosef 551) Walking by yourself(Beis Yosef 551),Rav Chaim Kanievsky says even to do a mitzvah (Nechmas Yisroel pg.58, footnote 161) and you should not walk between the sun and the Shade(Mishnah Berurah 551.102) He is described as being full of eyes scales and ears and rolls like a ball between the sunlight and the shade, Reb Shimon Bar Yochai says he has one eye on his heart and who ever sees him dies on the spot.
For more extensive research see: Pesachim 111b The disscusin includes the Morning Ketev is the"Ketev Meriri". The Afternoon Ketev is the "Ketev Yashud Tzaharayim,". and times of the year when Ketev are found and where he lives.
Intresting Fact from Rashbam,Pesachim 111b "Shaata": Rich people are at less risk of harm from Ketev, because their Mazal is good
Somehow I get the feeling it's connected to plagues of malaria caused by mosquitoes rising off the swamps of Galilee in the late summer. Think; Meriri is the sound a mosquito makes; it is described as spread by "flying demons"; the mosquito is used in the Tanya as an example of pure evil, because "it eats, but does not (visibly) excrete"