Interestingly, some writers have used spellbounded as a past tense form, suggesting that they view spellbound (not spellbind) as the present-tense form. For example, from The Postal Record (1985):
First District Congressman Charlie Hayes spellbounded the convention with his wit, poise and knowledge of letter carrier concerns.
From Mary Ann Harrell, Surprising Lands Down Under (1989):
I asked a veteran coach driver there what single spot the tourists liked best. "Ah, Mount Cook," he replied, "she spellbounded them when she came out!" Much of the time she stays in, in the long white cloud that often cloaks the highlands..
From Bill Pemstein, A Stone's Throw (2011):
On March 20, Stone again took on the Texas Rangers and spellbounded them for four innings. A six-run fifth inning drove him out. A two-run wind-blown homer from Bump Wills was the key blow.
From Stefan Dumitrescu, You'll Be Ether, Too, Beyond the World (2015):
The one who spellbounded the heart of famaous Iancu Delamare, merchant and lawyer who was proverbial for being a Casanova and a man with mysterious powers, was the daughter of a chief governor of Moldova, who had the home in Braila, because, having large estates, he was dealing with the grain, fur and fish trade.
From S. Vijay Palaniappan, Chocolate Sundae Fudge: The Embrace of the Entwined Game (2016):
She aid, 'To be kissed in the middle of the street in the pouring rain,' she amazed me by uttering this. I was spellbounded and was speechless for a while. Then I ardently said, 'I'll make that alive for you, my dear Rishita'.
Likewise, some authors have used spellbounds as the third-person singular form of "to spellbound." For example, from Joseph Kelner, Personal Injury: Successful Litifgation Techniques (1964):
The summation concludes with a lyrical story which spellbounds the jurors. Counsel impresses the jury with the importance of their task, while at the same time employing a subtle analogy. It is not difficult to equate the parties in this case with the people in the story.
From The East, volumes 18–19 (1982) [combined snippets]:
This passage from his writing on aesthetics shows how strongly he was attracted to Tokushima: "It's easy to find artwork that spellbounds. Just walk through Tokushima city and the surrounding countryside. The tools in a gentle line at the shop, a ceramic teapot, a winding country road, a working farmer—everything is a work of art."
From Herbert Loebl, Government Factories and the Origins of British Regional Policy, 1934–1948 (1988) [text not shown in snippet window]:
The leading article in The Times of 20 March 1934 - the issue which opened the series - saw 'a background of bare justice behind the unyielding Socialism which spellbounds Durham Man today, because he thinks he can see no proof of ...
From Aśoka Jeratha, The Splendour of Himalayan Art and Culture (1995):
The scene spellbounds the viewers capturing the lure of nature of the medieval times perpetuating with that of today. Its inner walls are the epitomes of the miraculous power of the beauty of line and colour, unfolding several chapters of history with manyfold sequences altercating with the modern times.
From India. Ministry of Information and Broadcasting, Publications Division, Freedom Fighters Remember (1997) [combined snippets]:
Shri Datta Mazumdar, still walks straight at the ripe age of 93, attends court and spellbounds all and sundry with his oration delivered in a stentorian voice. With a memory so sharp that would put any young man to shame, he still vividly recalls his days during the freedom movement and blasts off - "we have been cheated out of independence of undivided India!"
And similarly for spellbounding, from Adrian Jasper, "Deadwood," in Lady Muslim with a Pen: Poetry and Essays (2012):
Face like a sculpture, designed to a chisel./ A frame that wows, spellbounding with a sizzle:/ But “wow” turned to “whoa!” when he spoke, expounding the Ugliest sounds:/ From “spellbound” to “hellhound”...
And for the future form "will spellbound," from Ed Rosenthal, The Big Book of Buds: Marijuana Varieties from the World's Great (2011):
Named for the heroic woman in the Arabian legend, The Thousand and One Nights, Paradise's She[h]erazade will take considerably less time than the title implies, finishing in about 56 nights. In the legend, the character Sheherazade saves her own life using her exceptional storytelling skills that awe the King into sparing her life and eventually taking her as his Queen. The variety bearing her name will spellbound you with the promise of her potent, musky-sweet flowers, possibly making her the queen of your crop.
And for the infinitive "to spellbound," from Black Business Digest (1973):
We have covered a lot of ground in a short period of time. My attempt here was not to try to spellbound you with some highly sophisticated financial lecture; but to try and get down to some of the nuts and bolts or the nitty gritty of what makes bankers tick, and what you have to do, to turn your banker on ...
Many of these instances are not the work of great literary stylists—although the excerpt that speaks of "chapters of history with manyfold sequences altercating with the modern times" is pretty hard to beat—but they do suggest a parallel (and rival) series of forms based on "to spellbound" in opposition to "to spellbind." Whether that indicates a double layer of verbal defectiveness or the linguistic equivalent of a largely uninhabited house is unclear to me.