I don't understand why it isn't a good spell as written. Maybe I'm looking at it from the eyes of a 3rd level Warlock, but with a single attack roll I can continue to do 2d12 damage each turn automatically. Where's the bad in that? It's clearly better than a cantrip at that point, and if there's even a hint of a melee line preventing a rush at me, I'm golden.Please explain?
I'm away from my books at the moment, but here it goes: You have to hit with a spell attack. That makes it kind of all or nothing, but there is the chance that you will miss and no 'save for half'. When you do hit, the damage is okay (1d12 is pretty swingy), but you have to use concentration and your action to get the benefit of continuing damage in subsequent rounds. This could be nice, but given the largely short duration of 5e combats, tends to drag out the damage and the combat is usually over by the time the damage inflicted builds up. The target can avoid subsequent damage by moving out of range or getting full cover, or he can just move up and pound you to break concentration (granted, the target is unlikely to know how the spell works in detail, but trying one or more of these things is probably what the victim was going to do anyway). So, mediocre damage round by round at the cost of your concentration and subsequent actions that can be disrupted rather easily. Oh, and the combat is usually over by the time the damage starts to add up. Also, using a higher level spell slot only ups the initial damage, not the following rounds' damage.
So, it is not absolutely terrible, especially if you can contrive to have advantage on the spell attack roll, or cast true strike (which makes the set up and payoff take even longer, by which time your group will have already finished a short rest). It may be a good spell to break out on a single big bruiser type that your melee types are tangling with; if you can hit, you can keep pouring in damage each round without much risk of it being disrupted for only one spell slot. But, IMHO, it needs a bit more to make it live up to its billing. Maybe the target loses its reaction, like with shocking grasp, or has disadvantage on ability checks (this would probably be too much for a 1st level spell). This would really help convey the whole writhing on the floor until your dad disrupts the spell vibe (err, or something like that) and make it a more worthwhile choice.
