merlin stands for Mixed Effects Regression for LInear, Non-linear and user-defined models. The merlin packages implement the methodology described in the paper:
Crowther MJ. Extended multivariate generalised linear and non-linear mixed effects models. arXiv:1710.02223.
You can get the methods preprint here (or the easier to read software preprint here), which I recommend you read before trying out the accompanying software (note, merlin used to be called megenreg, and the syntax is out of date in the methods pre-print!
Simulation of simple and complex survival/time-to-event data
The survsim packages implement the methodology described in the papers:
Crowther MJ, Lambert PC. Simulating biologically plausible complex survival data. Statistics in Medicine 2013;32(23):4118-4134.
Crowther MJ, Lambert PC. Simulating complex survival data. The Stata Journal 2012;12(4):674-687.
I recommend you read them before trying out the accompanying software. This page details the development of the package, both in terms of version histories and planned features to be added.