What to do? Start SMN or Diabolos first?Follow

My vote goes to no.
He's useless until 65, and even then, and at 75 I still consider him useless, due to Merit Blood Pacts. But, that's just my personal opinion...

Honestly, I can scarcely even remember the last time I called Diabolos. Phalanxga is nice, but often I find im more called on to use Ecliptic Growl or Earthen Ward thanks to the heavy NIN-subbing shadow-users I team with. In instances where you use Utsusemi (which account for a good portion of higher level parties) Stoneskin beats Phalanx beacuse it lets you recast while being hit.

He has one attack worth mentioning, and even then it's pretty unmentionable. Nether Blast is a fairly consistent attack regardless of what level you use it, but the low-end damage on it makes it a poor choice over your far beefier level 70 physical blood pacts on anything which doesn't have high defense or evasion. It's an ability you save for the physically resistant mobs rather than a catch-all winner.

Once you get level 75 and access to the even more powerful merit magics you'll probably never touch that Nether Blast. Especially when you can blast out a nice 1.5k on a magic burst (Yeah, I realise no one really chains any more, but it's still a Diabolos-beater).

He does have one use though. His unique 2-hour skill, Ominous Ruin, can very occasionally hit for incredible damage. I would like to stress, however, that as it's a HP-percentage based attack it often hits for a very small percentage. Still, one out of every few hundred is still a chance, right?

What everyone else here has said. You can afford to wait later, much later, heck wait until your SMN is 75 before you go for Diabolos. I doubt anyone is gonna care that you leveled without him.

He's still a good avatar to have around though once you get into endgame, mainly due to Nether Blast. I had a lot of fun Wednesday night chasing down Kirin and blasting him like there was no tomorrow. *evil laugh*

Still working towards my merit BPs though. Must...reach...level 5...Wind Blade. (Hey, merits come only at a decent pace when you campaign for them.)

I use Diabolos to buff BLMs.

Out of curiosity, why Diabolos in particular vs the other summons?

Somnolence, despite being a Ward pact, is a direct damage nuke. As such, it will give the same chance at a skillup as, say, Fire II or Thunderspark, even if resisted. Other Ward pacts are hobbled either by the self-buff skillup calculation, or by needing to have an effect on your target to give a chance to skillup.

The addition of Nether Blast as a Rage nuke also helps in cases were you don't want/can't switch out Avatars.

You won't have many major problems if you wait until 75 to get him. Mostly the use for him relates to high level, situational uses.

High level stuff does use Diabolos -for instance my petshell was fighting Gration yesterday and we tried using Ifrit and Titan first because of elemental weakness, then moved onto Garuda and Shiva just to try something else, and finally Diabolos and Fenrir. Now Diabolos' Netherblast did the most damage averaging about 300-400 per BP. Fenrir managed anything from 300-500 with Ecplipse Bite but wasnt as consistant, probably down to the fact that EB is a multi hit BP. Big difference between the two was that Diabolos is so much more MP expensive than Fenrir - and when you are fighting for 20-30 minutes, even with RDM in your party, MP does run very low.