I actually never NEVER NEVER upgrade Dagon when I'm playing Nyx until it's really late in the game (or unless I feel like spamming #dagonop), and here's why:
Dagon 5 total cost is a whooping 7980. For 5200 (7980 - initial cost of dagon) you are doubling its burst damage and pretty much halving its mana cost and cooldown. And that sounds pretty fucking cool when you think about it...
BUT
Instead of spending 5200 gold on upgrading it, for 4900 gold you can get THIS thing
You get not only more raw stats out of it (10 int compared to the 8 int you gain from fully upgrading dagon, 7 more strength, and 40(!!!) agi); but also MOAR DAMAGE ON YOUR COMBO!!!!
Level 16 Nyx has 56 Agi (1 level of stats), which become 99 with Dagon 1 and Ethereal. So, your Ethereal blade active is dealing 273 damage, on top of decreepifying them; which in turn increases the damage of your abilities by 40%!
A Dagon 5 Nyx effectively has 641 (Vendetta + AA) + 260 (Spike) + 800 (Dagon 5 ) + 265 (Mana Burn (
I'm assuming 53 INT. All heroes combined have an average int growth of 2.05, and I assumed 20 starting INT which is rather low, actually)) burst damage; so min
1966 damage, may be higher if your target has more int.
Dagon 1 + Ethereal Nyx deals 681 (V+AA; not amped of course) + 273 (Ethereal) + 364 (+40% Spike) + 560 (+40% Dagon 1) + 371 (+40% Mana burn, assuming 53 INT. Again, this one can go up); for a grand total of
2249 damage.
Keep in mind that, since Nyx is an AGI hero, your attacks will deal an additional 40 damage from the Ethereal Blade too. This on top of decrepify making you immune to autoattacks so carries won't three shot you. Spells will hurt like a motherfucker tho but you have Spiked Carapace for that ~
TLDR: Dagon 1 + Ethereal > Dagon 5 on Nyx
EDIT: Sorta ninja'd by Poppy but I has math c: