Alriiight, lets talk about this thingy, because i think its very interesting. When i first saw this card i thought it was pretty great until i saw the "instants or sorceries" clause, instead of just spells, then i was pretty down on it, but the more i think about the card, it seems more useful.
I know some people, and myself, are often not that high on overly comparing spoilers with existing cards ("its just X with the expection of Y") because this often results in an oversimplificatation, however i will use some comparisons here. Keep in Mind i do not want to push a this "its just..." conclusion at all. This card is its own thing.
first of all:
the damage scales quadratically with spell count
This is not quite true, scaling quadratically would mean that the it would output i.e. 36 for the input 6.
What this does (just like aetherflux reservour) is summing up all the numbers from 0 to n, where n is the number of spells you cast this turn. The sum of this (the total amount of damage done after casting n spells) is output by this common function (small gauss) f(n) = (n+1) *n/2
Alright with that out of the way i can see this card used in 2 archetypes:
Storm-like (or light, with witch i mean possbile decks that use this card over the cause of maybe 2 turns) Decks that try to cast as many spells as possible in one single turn to end the game right there and on the other hand Xerox style controlling Decks witch focus on hindering the opponent while constantly pinging them over a large number of turns.
In Storm this comes down to the differences between a "upstream" (win cons that you play before stringing a lot of spells together) and a "downstream" ("""""after""""""), finisher in this kind of Decks. I thought about this difference a lot when Aetherflux came out. With a mostly pure wincon like Aetherlufux Reservoir i had this to say in another Thread:
Its a win con you have to plant before or as you go off as opposed to the finisher after it. As such it is not a absolutely dead card in your hand, when you are not about to win, and can provide some marginal lifegain over a few turns.
Planting it a turn early before going off also frees up the 4 mana, which is not possible with Tendrils, but makes it pretty vulnerable.
Beeing colorless makes it much easier to cast of course, but beeing an artifact makes it very vulnerable and way easier to handle than when Tendrils hits the stack.
Having to play it before or while going off, makes some lines of play, that work for a tendrils finish storm decks more risky. If you don't have the card in your hand, trying to find it while going off might result in you finding it too late and not having a critical mass off spells left to trigger it. This can be played around however (holding some not needed Mox opals and such), and the card counts how many spells have been played before it hits the battlefield, so playing it while going off, will still give you a ton of life.
A lot of this applies to this card as well, however it is immune to Null Rod/Stoney Silence, Revoker and other such effects, since it is not an activated, but a triggered ability. In short i would say using an upstream wincon like this produces more risk, but allows for easier combo turns and some freedom of how and when to combo out. When iam talking about a downstream win con in Storm i am ofcourse talking about Tendrils.
Since this card does not trigger from artifacts but just from instants and sorceries i do not think this is suitable for your everyday PO List, in which i believe Aetherflux would mostly be at home if it would see any play. That leaves Ritual Storm lists, with their high inst/sorc count. This Deck often relies on artifacts to produce enough storm for a lethal tendrils, but remember we do not need a storm count of 9 to make this card lethal, but just a count of 6.
The question is if that 3 less storm needed does actually counteract the loss of artifacts as a "storm" source. Iam not sure about that. My current list plays 11 dedicated fast mana artifacts and 7 dedicated fast mana inst/sorc. If i assume to use about 4 fast mana spells in my combo turn to achieve lethal tendrils that means iam using about 1.6 non artifact fast mana spells in that turn. By that very crude and possibly (certainly) incredibly oversimplified calculation you would only lose 2,4 Storm which seems to be less than 3. So needing 3 less storm does seem to counteract the loss of artifact stormcount. That is ofcourse ignoring that decks playing this as a main win con might opt to cut something like LED for another CabalRit.
Now since storm count doesnt seem to be huge problem, lets see if the added vulnerabiliy i talked about above (artifact destruction, opponent "seeing it coming", easier to counter than an actual Tendrils.) does overshadow the inherant bonus this card had over Aetherflux Reservour: The possiblity to use this as removal over the course of some turns or at the beginning of your combo turn to clear the way. This is really the huge factor that sets this card apart from Aetherflux or other upstream win cons (like Mentor, which is harder to deal with, but very much slower and does rarely have the potential to win the game in the same turn it was played. I do not consider Mentor a upstream win con that has the potential to usurp Tendrils). Using this as the threat of removal, over a few turns or in the same turn you plan to go for the win, is the real strengh of this card for sure.
Possible targets you would want to remove are plenty: Thalia and other taxing creatures, Revoker, Spirit of the Labyrinth, Eidolon of Rhetoic, Kambal , Scab Clan Berserker... just to name a few of the ones that are actually stopping you from going off. In addition this can be used before actually winning to just buy time and control a bit. Targets here would be aggro shops creatures, Delver, Mentor and Pyro as well as their tokens...you get the point.
I did not name cards like Etherswon Canonist since with its 2 life pared with the ability is immune against this kind of removal (Pack your sideboard with that bad girl if you are scared of Sentinal Tower), and Leovold since the card is absurd and should never have been printed in the first place.
You could also ping away some orchard spirit tokens but that doesnt really seem sustainable since you would need to consistantly be able to cast an instant at your EOT. Maybe it buys you a turn.
Wow, having your Win con in a neat package together with removal seems to be a huge deal, however keep in mind that playing this with the intential to control for a turn or two does open you up to lose your win con pretty quickly: Any artifact removal or Dack will shut this off, and just slamming it out there to remove a pyromancer and then losing it to something like that does not seem worth it.
This approach however seems to be pretty neat in a xerox style deck to be honest. Playing a xerox Deck with control elements, countermagic and other removal (you all know what kind of shell iam talking about) could absolutely make this to a must remove threat. Imagine your cantrips to double as removal and then flip over to be a win con if you need it. This can go on over a number of turns with you defending it, effectivly making this a less powerful but way cheaper Sphinx bone wand (my favorite casual control win con) . 4 generic mana is not really a problem if you ask me in such a list. Is it way easier to handle than your usual win cons in that deck? For sure! But its also not just a win con, but a control element, turning win con when the time is right.
As for Storm Decks i think this can very much be the perfect secondary win con. I wouldnt cut my Tendrils, its just way safer and harder to handle, however i can totally see playing this as a second win con maindeck. I see people playing stuff like a second Tendrils (not a fan at all!), Empty the Warrens or Mentor as secondary win cons. I think this is just better. Having the old reliable hard to interact with, extremely powerful finisher in Tendrils and a, above all flexible, secondary in your Deck seems pretty cool.
I will definetly try this as a one of in my Ritual Storm Deck and maybe brew some other decks that are more centered around this (even though i believe this card is best in low numbers).
I love this card design, its suprisingly deep, lends itself to different play styles and functions in different shells, has a lot of potential and is still easy to handle and not over the top. Very interesting card.
Edit: @Serracollector said:
Neither Null Rod/Stony Silence or Revoker shut this off
Totally missed that, that makes this card a good chunk more resiliant, i removed any mention of that interaction in the post.
Upside: Very flexible as a removal/win con to different degrees in different shells.
Downside: Not very resilient, easier to handle for most Decks than usual win cons.
Has potential in xerox decks and ritual storm decks, mostly as a secondary win con.