All of the master spell quests are notorious for being buggy. The best option is always to attempt to re-load your save and try again.
DO NOT ATTACK the Draemora Lord if dialogue doesn't appear- THIS WILL BUG THE QUEST.
Without knowing what you've tried so far, here's a short list of solutions that may fix your problem:
First, make sure you don't have the summoning stone in your inventory, as even if the quest is bugged at the summoning step, you only need the stone to complete the quest. Make a new save, reload, and then go through all of the quest dialogue options with Tolfdir. If that doesn't work, reverse pickpocket him and he should continue the quest as normal. See this thread.
The most likely issue is that either you made your conjuration skill legendary, or the game thinks you did. This will result in no dialogue options being presented to the player. The fix is to reload your save, or raise your conjuration skill above 90 again (this may not work- you may have to revert to before you made the skill legendary). Alternatively, reload to before the summoning, talk to Tolfdir again and go through all of the dialogue options, then retry the summoning.
The most common issue players in general have had is being too far away from the Lord when he spawns. This can cause the dialogue prompt to fail,and can be fixed by reloading the save and trying again closer this time. Be wary of followers attempting to attack the summoned Draemora immediately.
From the wiki:
"Followers may attack the Dremora immediately after it is summoned, allowing no dialogue to pass and preventing the Dragonborn from re-conjuring it.
Instead of submitting to the player after being defeated, the Dremora may die and be lootable. He will also be unable to be resummoned. "
- The solution here is to either leave your companion at home while attempting this quest, or to summon an atronach/other draemora and instruct your follower to attack it while you summon the Draemora Lord. Certain quest followers (Esbern, Delphine, Vilkas, Barbas) will be immediately attacked by the Draemora Lord, so consider finishing the offending quest first. (This is what broke it for me- finishing the interfering Companions quest allowed me to complete it).
"Various actions may cause the Draemora Lord to be killed and become lootable"
- The only solution here is to reload the save and try again. Be wary of the paralysis and disintegrate effects in particular, as those seem to cause the most problems.
Unless you have a previous save available it looks like you're stuck. Even if it means losing a fair bit of progress I'd say it's worth re-loading a save. The master conjuration summons are very powerful.