Something I'd like to note after playing with Delin, I used an extremely high initiative hero (Gallowman with multiple layers of celerity) and as a result Delin would unspawn before moving. Nevertheless, by casting grave elementals on top of Delin, I was able to give Gallowman repeated gravewards, making him near immortal (granted the real power here was Gallowman's consume spirit, which paid for all this.)

It's not effective at all.

In fact, I would go so far as to say it's utterly useless, since it costs 500 mana and he'll go away after 3 turns.

Also, as an addendum, The summon line really needs the summons to be re-organized, because most of the most powerful summons show up quite early.

For Empire, summon skeleton is a free sacrifical summon every battle, very useful.  And summon skeleton horde is ridiculously overpowered with the swarm bonus factoring in.  The biggest thing is that the AI likes to focus on the skeletons, which basically prevents damage to your actual army.

For Kingdom, summon wisp is a critical unit for healing and damage.  The lightbringer, however, isn't very good since it has 21 physical attack.  It does make a useful tank, but ultimately, it's completely outclassed by summon skeleton horde.

And on the main line...

- Summon air elemental is by far the best summon due to it's high initiative, lightning damage, impale, and spell.

- Grave elemental (in my experience) is worse than the earth elemental.

My suggestion would be:

Ice elemental --> crag spawn --> grave elemental --> earth elemental --> air elemental.

I'd agree air elemental is more useful than earth elemental, by the way. Air elementals can get taken down pretty easily by more powerful units later in the game because of the zero defence, but the higher move and initiative mean they get involved in battles more, and in practice I don't mind casting the summon again because I'll often get a higher level unit because of more shards or other bonuses.

I've not played with Delin but 500 mana to (at best) win one battle seems way over the top. Maybe 250 mana and have him last for five turns? I wouldn't mind paying 250 mana later in the game for a more or less guaranteed battle win.

If the question is whether I would prefer to tactically cast summon grave elememental (with bonus 1 radius reincarnation) with cast time of 1 turn for 60 mana or summon delin with cast time of 2 turns and 500 mana then yes, summon grave elemental wins every time. I'd agree Delin is too expensive. The only real question is what would make him more useful. I've suggested halving the mana, you've suggested dividing it by 8. I've suggested making him last longer, you've suggested reducing the casting time. In terms of the roleplaying side of things, I think it would be quite cool to have to wait to summon the demon lord but he then wipes the floor with your enemies. I might pay 250 mana to have a more or less guaranteed battle win, but he needs to last more than three moves. The amount of mana could be reduced further if necessary.

I'm not quite sure what you're getting at when you're talking about end-armies. There are lots of different end-armies which will wipe the floor with almost anything. The question is surely under what circumstances, with a less than all-powerful army, the "ultimate" summons are useful. I agree the cost of summon delin needs to be reduced, I just wouldn't go quite as far as you. Not until a smaller change had been play-testedt, anyway.

Increase his upkeep to 5 mana, and it sounds like winner.