That's a college try for sure! Two things are working against the annotated short-form of this monster.
First, I think A5E in particular - where that stat block comes from - has a more complex design (compared to 2014 5e) that makes shrinking stat blocks more difficult. Ambusher, Ruthless, and Acid Spite are examples of this increased complexity. I think the design gestalt of A5E works against this kind of extreme condensing.
Second, legendary monsters are always going to be lengthier; even in short-form by at least 4-5 lines.
If we imagine complexity as a dial, with this sort of complex monster we can turn it up to a 3/4 page stat block (iirc that's what the A5E ancient dragons take up) or down to a small paragraph. However at both extremes, there are downsides. Too condensed and there's information / clarity loss. Too lengthy and it's hard to keep track of it all.
Anyhow, I'll take a stab at a short-form / in-line text / AD&D stat style version of this
A5E Ancient Black Dragon...
View attachment 378377
Ancient Black Dragon. LE Gargantuan;
Senses 22 (blindsight 60 ft, darkvision 120 ft);
Stealth +9 (advantage in water);
Lang Common, Draconic;
AC 22;
HP 367;
Spd 40 ft, fly 80 ft, swim 40 ft (amphibious);
Saves S +8,
D +9,
C +14,
I +3,
W +9,
Ch +11;
Defenses Acid resistance, Legendary resistance (3/day);
Multiattack (x1 bite or acid spit, x2 claw) x1
Bite +15 (15 ft) 4d10+8 piercing and 2d8 acid (grapple if target can’t see dragon), x1
Acid Spit (60 ft, DC 22 Dex, 4d10 acid and 2d10 ongoing/half), x2
Claw +15 (10 ft) 3d8+8 slashing;
Special Attacks Acid Breath (re5-6, 90 ft x 10 ft line, DC 22 Dex, 19d8 acid/half),
Reaction Tail (when creature within 10 ft hits dragon; tail +15 3d8+8 bludgeoning and push 10 ft),
Ruthless (1/round after critical hit make claw attack);
Spells spell save DC 19
3/day
fog cloud, pass without trace, legend lore, speak with dead
1/day
create undead, insect plague
Legendary Actions (3)
•
Darkness (40 ft sphere magic darkness w/in 120 ft)
•
Roar (120 ft burst, DC 19 Cha, frightened, repeat save)
•
Wing Burst (15 ft burst, DC 22 Dex, push 10 ft and prone, then dragon flys 40 feet)
CR 23 (50,000 XP)