To be honest, mystery author, you had me right from the start. Your opening line is quite possibly my favourite opening line of all time: Horace Slughorn propped one leg up on the coffee table and sipped his blackberry brandy. "I won't be your boy toy again, you know." Fantastic. It's funny, and characterful, and sets the scene for so much of their interaction right from the start: the attraction, humour, trust/ditrust, honesty/subtefuge, shifting balances of age and power. I can just hear Horace saying it, too <3
I love how aware Horace is of what is happening, yet he can't stop it, or himself: Horace shivered and closed his eyes. Albus still had so much power; it was still intoxicating. If he wasn't careful he'd be bent over a sofa and shagged senseless before he knew it. Funny, and delicious at the same time.
Also, it's great that he is wary/wise of Snape, throughout (considering the sad conclusion to the 6th book) - that's a clever theme to sow from the start, with the ponderings about the potion cupboard, and Horace's jealously of Albus' relationship 'with the younger man'.
From there on, you kept the sparkle going in spades: the public sex scene is hilarious, and spot-on in terms of character (Horace handled it with aplomb, of course), and I love the dynamic throughout where Albus is actively chasing and making - surprisingly bold - sexual overtures, and Horace likes being chased; it appeals to his slightly submissive side.
The end is serious and sobering, with a light, deft touch. You lend great respect to Horace, here, as he pushed his shoulders back and joined the others holding their wands to the sky. We really feel for him here, and it makes a fitting end to the fic.
Thank you again for writing this, Mystery Author. The pairing is a favourite of mine, and I'm delighted that you wrote it so very well :-)