Hello,
I’m looking for a simple way to use one filter on several tracks at once. Is there a way to group track into 1 effect track or something similar?
Otherwise, I thought about using Hydra but it’s used for modifying several parameters from different track DSP into 1 slider, while I want 1 parameter from a filter DSP to affect several tracks.
I hope it’s clear enough. 
The traditional way I guess is: put send meta devices on the tracks, routing them to a send track with your filter DSP.
Alternatively since v2.8 create a group track, move your tracks to it and put the filter DSP on the group track.
Edit: Yeah, Cas is right, of course they aren’t meta devices.
Depends on the routing you want to end up with… If you want say 3 tracks to go through the same LPFilter, you can easily 1. group these channels (by alt-dragging them) and put/move the filter dsp on/to the group channel or 2. send their signal to a send track with send devices (it’s actually not a meta device but whatever), then put the filter on the send track (on the right of the master track, you should have seen at least one because renoise always starts with one (try File->New Song (no template) if you don’t have em) )
anyway if you want the channels to have different routings/fx after the shared filter dsp you will need to keep them all on their own channels and use the hydra. it can still be handy organizational-wise to have the channels in a group and put the hydra device on the group track so that it’s evident that you control group track stuff with it.
If you don’t need to return the individual signals to their origin source, you can use one single send track with a filter on it. Send tracks are the tracks right of the master channel. Just drop a Send device on every track you want to filter and route all the send devices to the Send track, where you put the filter on. You can automate this single filter then.
If you need to keep all your signals on their individual track, because you want to add reverb, compression or what ever here and there, your thought of using a Hydra was absolutely right. You’d just have to drop a filter device on each track you want to filter, and then you connect all these filters to the Hydra. Once you’ve done so, you can change the params of the multiple filters, using the single Hydra input slider.