First, I need to establish the purpose of the book. Is it a memoir? A political manifesto? An apology? Mengistu might be presenting his side of the story, justifying his actions. Alternatively, he might be revealing new information. The title "Tiglachin" might refer to a historical event or a concept. If I can't find its exact meaning, I might have to make an educated guess based on context. Maybe it's a term from Ethiopian history or a reference to a struggle against colonialism or internal conflicts.

I need to address why he's writing this book now. Retired leaders often publish their memoirs to set the record straight or apologize for past actions. Mengistu might be seeking redemption or presenting his version to those who have been critical of him. The Ethiopian government's stance on him could be an important aspect—how do they respond to his book? Do they try to suppress it, or is there a change in attitude over time?

I should also consider the historical and political context of Ethiopia around the time of the Derg regime. The Ethiopian revolution in 1974, the fall of Haile Selassie, the rise of Mengistu, the Mengistu era's policies, the Eritrean conflict, the famine, the Red Terror, and the eventual fall of his government in 1991. The book might address these events.

I need to include the structure of the book into 25 installments. Each installment might focus on a year, a political event, a personal experience, or a theme. The PDF format could mean it's available for free or for sale online. There might be discussions and debates in the digital sphere as each part is released.

Potential challenges in writing this story include avoiding bias, accurately representing both sides of Mengistu's legacy. It's important to present the Ethiopian government's perspective on his regime and the opposition's view, as well as his own narrative.

Also, considering the diaspora's role. Many Ethiopians in the diaspora might have strong opinions about Mengistu, depending on their backgrounds—some might support him, others would oppose. The book's release could polarize these groups.

Potential plot points: the announcement of the book, the first installment's release, mixed reactions, subsequent chapters building up to more controversial topics, perhaps a chapter apologizing for past decisions, a chapter addressing personal regrets, the final installment concluding with a call for reconciliation. There could be interviews or statements from Mengistu included in the PDF, or discussions of his current views on Ethiopia's development.

Including quotes from the book, if any. Since it's fictional, I might need to create plausible quotes that reflect Mengistu's possible stance. For example, in the first installment, he might introduce his reasons for writing, address the Ethiopian people, and set the tone.